That's a somewhat fuzzy question. Depends on what you want in the end. You could measure from the axle to the underside of the fork crown and then match that measurement on your new fork, or you could measure from axle to top of tire, add about a cm for clearance and go with that. If you match the old fork dimensions, your steering won't change much, if at all. If you go with the shorter legs your head tube angle will get steeper and your steering will get quicker.
Looks like that site offers all of the stuff you need to do that conversion, but frankly, at those prices I'd just buy a good used road or cyclocross bike. That brake setup looks sketchy to me, as well. Seems a little like turning a station wagon into a pickup truck. Sure you can do it, but is it really worth it? If I still worked at a bike shop and had all of these parts laying around used I might do this, but I sure as hell wouldn't spend anywhere close to a thousand dollars to wind up with a frankenbike that isn't well suited for anything but commuting and coffeeshop runs.
FWIW, if you just GOTTA do it, you can find a cheap steel 700c fork with canti mounts pretty easily through an online retailer, or if you keep an eye on Craig's List you may find a used carbon fiber cyclocross fork with canti mounts. You'll probably need it to have a 1 1/8 steerer, and if you buy used make sure that the steerer tube hasn't been cut too short for your intended frame. Google "700c fork cantilever mounts" and you'll see a shit ton of cheap forks that will work.
i just wanted to know how to calculate for the rigid fork length .
so i can get rid of the rock shox[/Quote]
In that case, just measure the diameter of your intended wheel/tire combo, divide by 2 add about a cm, and use that as your axle-to-crown dimension. Find a fork with that dimension and you're all set.
just measure the diameter of your intended wheel/tire combo, divide by 2 add about a cm, and use that as your axle-to-crown dimension. Find a fork with that dimension and you're all set.[/Quote]