I have an opportunity to buy a second-hand 2008 Reba Race (100mm) which has an Axle – Crown measurement of 510mm. The current rigid fork A-C is 490mm. Will the Sag on the Reba even up the difference? The Frame manufacturer specifies corrected for 80-100mm

have a look on the cotic site under geek,, it shows geometry change with fork length,, 26inch bikes

your sag should be 25 to 30 % of the travel so that brings you back to 485 ish i cant see a problem,,

just check what fork offset your frame is designed for

Will be absolutely fine.