-
Measure the a-c of the suspension fork, preferably while sagged.
If you can't measure it while sagged, measure it fully extended and then subtract 25-30% of the total travel.
Bear in mind that the frame will be designed to work within the range of travel, so if you get a rigid fork with slightly less a-c (but not less than the suspension fork fully compressed), it should still steer within the boundaries of acceptability.
when going from an old rockshox recon 351 fork to a rigid fork. Are there any rules for axle to crown measurement?