This is a question that causes considerable heartburn every now and again, not least because it isn't obvious from which end the bike-specifier should unravel it. I've deliberately left out the numbers and concentrated on the relationship of the components in 3D space, and I end with a decision tree which I hope will make the troublesome procedure easier to visualise and grab hold of, even for novices.
A bottom bracket is distinguished by a number of specifications. Let's just take the most common kind, which is referred to as a "square taper", which is the type of mount it provides at the ends of the axle through the bottom bracket to the crankset, which must then also be a square taper crankset.
First, there is the width of the shell into which the BB fits on the bicycle frame, which is usually either 68mm or 73mm on a touring bike, this being the distance over the threads on the inside of the bottom bracket shell, matching the threads on the outside of the bottom bracket itself. You should have no trouble with this because the designer of the frame fixed this length and it is given in the frame spec. Methods of fitting and fixing an eccentric bottom bracket (used for adjusting chain length) don't complicate the ends of the BB axle, and indeed simplify it because the frame designer in most cases species the best EBB by brand and model number.
Secondly, the axle which has the square tapers at its ends, has a length, called the bottom bracket axle length. This length is chosen to put the chainline in the right place (old-timey road racers also used it to fix their tread width, now sometimes poncily called "the Q-factor"). The chainline is specified as a laterally perpendicular distance from the fore-aft centreline of the bike. On a Rohloff installation it is assumed to be within one millimetre of the Rohloff specification, which differs for the kinds of Rohloff sprockets available. (There are three different recommended chain lines, but the one which pertains to the 13 tooth sprocket can be ignored for big-wheel touring bikes.) Working back from the desired chainline for your Rohloff sprocket fixing method, you need to know the offsets of the chainring fixings on your chosen crankset, usually two, sometime three possibilities.
You also need to know if you will use nothing to protect the chain, a Chainglider, or a bash ring. If a bash ring is fitted on a compact crankset with only two sets of fixings (for two chainrings), clearly the bash ring goes on the outside and the single chainring goes on the inside. A bashguard can also go on the outside on an outer ring with longer chain bolts and spacers, if the cranks allow it. The Thorn ones are advertised as doing so. If you're fitting no protection or a Chainglider, the chainring can go on any of the fixing positions and you have more flexibility in your axle length choice.
Whichever is the case, the centreline of your chainring must fall on the Rohloff-regulated chainline or within one millimetre of it.
These elements together then determine the bottom bracket axle length. Do not just assume that the bottom bracket axle is symmetrically placed in the bottom bracket: there are known examples where from one BB axle length to the next the axle format switches from symmetrical to adding all the axle length increment on one side, making the axle asymmetrical, that is, one that will not move your chain line, or will move it twice as much as you expect. Measure the BB when you get it!
In practice, you work it out iteratively until you get frustrated, and then ask in the community what BB axle length everyone else uses with which crankset, and double-check and triple check the numbers before -- you accept the bike-maker's recommendation. Also, in practice, while in theory we assume the BB axle is symmetrically fitted, a small asymmetry -- 2.5mm on my bike if I remember correctly -- will not disturb your riding pleasure. These spacers are fitted on the BB axle next to whichever end of the bottom bracket shell on the bike requires it to move the ends of the axle in or out to make the desired chainline. An eccentric bottom bracket may also be shifted to be off-centre laterally by a small distance in its holder or directly in the BB shell on the frame to make a final small adjustment.
There are also spacers available (some used to come with a new Rohloff box and may still) that you can fit on the crank to move the chainring a millimetre or two sideways.
The next consideration is the PCD or chainring bolt diameter on any crankset you choose. Remember from near the beginning, your bottom bracket axle ends and the holes in your cranks must match each other, but not all PCDs are available for all bottom bracket-crankset-mating "standards". I started out with a square taper because it is the longest-existing and most widely applied standard, and won't screw you down the line as one of the many Shimano "standards" has locked Dan in, preventing him from making a much-desired improvement without first spending an inordinate amount of money on what should be minor peripheral considerations, but in his case, because of his bottom bracket-crank-mating choice are not. Your crankset choice also has bottom bracket axle length implications, in that not all cranks curve out to the pedals the same distance. Martin's classic TA cranks for instance angle out very little and are desirable for other historic and aesthetic reasons, but hugely troublesome in having a "historic" PCD for which it is tough and expensive to get chainrings today, and tomorrow probably impossible. So you should choose your chainring before your crankset and your bottom bracket axle. Again, you need to ask whether you will use a Chainglider, because it presumes a thin chainring, preferably steel (think BMX as John R has already advised) or stainless steel (Surly); it means you must forego the very attractive Thorn reversible ali chainring because it is too thick for the Chainglider.
So, starting from the back, choose your chainring with a number of teeth that will suit (in cooperation with an available Rohloff sprocket) your desired riding style and your desired chain protection, a crankset with a PCD to match the chainring, a bottom bracket axle to match the crankset and the required chainline, and adjust all this more or less symmetrically on the only suitable bottom bracket axle length (bottom bracket axles are available in length intervals of two or so millimetres) with thin spacers directly on the BB axle and/or between the chainring and the crank.
Good luck with getting your chainline just so!
THANKS FOR CONTRIBUTIONS TO JOHNR AND PH.
