http://www.machinehead-software.co.u...engthcalc.html
Good luck. You have to be super accurate when measuring your chainstay lenth, and the recommended chain length should be really really close to spot on. If it recommends you to use 48 gear inches, a usable unrounded result should be something like 47.95 or even closer to 48. I used this calc to fix my vert drop road bike, and just ended up getting really lucky, since my first estimate with the calc was actually wrong. I just tried a different chainring for the hell of it, and it worked out. Also, check Sheldon Brown for some ways to get around vertical drops.