Disc brake or caliper?
With calipers it's usually the arch of the brake that restricts tyre size rather than the fork clearance, the difference between that arch with and without a mudguard is a couple of mm.
if the only restriction is the fork crown, measure it, just take a tyre off and accurately measure the clearance, subtract what you think is a minimum clearance, 8mm for me, though I'd prefer 10 - 12. Then you have to hope the manufacturers claimed size has some relationship to reality, though they're more likely to come up small than big.
Also - use = gravel bikepacking - conjures up some vivid images, but they might be different ones for different people! Are we talking light off road, or just short of technical single track? Racing around with minimal kit, or touring around but not quite touring? Whatever it is, have fun.