FYI, axle-to-crown for a suspension corrected rigid fork should be taken at 20% (to 30%) sag from whatever suspension fork you have.
What does that mean? Measure A-t-C, then subtract 20-30% of the fork travel, depending on your desired handling characteristics. 25% is a good guess, if you have no strong opinion. A 100mm fork is minus 20mm, a 120 is minus 24 (nearly an inch...), etc, etc.