So for the coming winter months I'm contemplating a fork change going from a suspension to a rigid, the bike is a Cannondale trail 7 (29er). For the type of riding I do, I'm fine with or without suspension but I really want to give it a try rigid.
My question is how to go about getting the right length for this particular bike whether I get carbon or steel. I've noticed most of the axle to crown lengths range from around 480mm to 490mm but then there's a description of "Suspension-corrected for 100mm".
Are measurements taken somewhere in the middle of the fork's range of travel? The fork on the bike measures around 480mm from axle to crown fully extended.

Any advice you can throw my way would be greatly appreciated!