Hi, I am 181cm tall with a standover (in riding shoes) of 84cm and Thorn chose a size of 561L for me (I bought an RST) and it is a perfect fit. I am in Australia and trusted their sizing methods. When I am inbetween sizes on bike frames, I go for the slightly larger frame, as I prefer putting on a shorter stem to reduce "reach" than have too long a stem if I went to the smaller frame size. I always worry with a smaller frame of being too hunched up. Just my personal choice.
On other bikes that you have owned, is your personal preference for a slightly larger frame or smaller frame if you are inbetween sizes ? Also, are you going comfort bars or straight bars ? as there is a bit of "reach" adjustment in the comfort bar, by 'rolling' it forward or back a bit to increase or decrease the distance between the handle bar and the seat (if that makes sense).
cheers,
Simon
ps: stem is 110mm