I'd be concerned about doing that...

Put a Clyde on a bike with an inexpensive coil-spring fork (Suntour SF7-XCM) and you may end up with more sag than ideal. Duplicating that non-ideal fork length on a rigid fork probably isn't the best way to go.

I'd suggest measuring the axle to crown race distance with the rider off the bike then subtract 20-25mm (20-25% of the 100mm fork length a.k.a. the ideal amount of sag). I would use that as the "ideal" rigid fork length. Given a choice between a fork that was slightly longer than ideal and one that was slightly shorter than ideal, I'd buy the longer fork.