Axel to crown is the first measurement you will want to take. For example the Marzocchi Corsa Superleggera (80mm) that comes on the new 26&6 measures 451.5mm and the 100mm version is 471.5mm. If you factor preference (angle changes) and sag you should be able to get the correct fork for your ride. The Mountain Cycle Carbon Forks are 100% carbon-fiber (steerer, crown dropouts, legs) and is available for 26" and 29" bikes. The 26" measures 451mm A2C and that is pretty perfect for a 100mm conversion (factoring sag). the 29" fork measures 483mm putting you really to slack (IMO) for that bike.