The ones on my current project are 77mm. You should check on the specs for your brake, rim, and fork. You can do a tiny diagram showing where the studs need to be to allow the brake to work properly. Or try the rig out on another bike, if you have one available to see whethere they work, then copy that. With the V-brakes it may be adjustible enough, but cantis often just don't work when set up on a fork, if the arms aren't wide enough.