The following was lifted from the article Making your own 29/26 that appears in issue #127 of Dirt Rag Magazine:

Take a pencil, a piece of string and a tape measure. Tie a length of string to the middle of the pencil. With the pencil placed in the fork dropouts, stretch the string taught to the surface of the fork where the headset crown race sites. Mark that point on the string with a pen and measure the distance from the center of the pencil to the mark in millimeters. This is your axle-to-crown length (L). If measuring a suspension fork, subtract roughly 10% of the overall travel form the overal lengtrh to account for suspension sag.

That’s how to take the axle-to-crown measurement on your forks. If you’d like to read the whole bit of how to turn your 26 inch mountain bike in to a 29/26 hybrid, be sure to pick up the latest issue of Dirt Rag Magazine.