here's a snippet from APE's site:
http://hayabusazone.com/drivechain.htmlWhen extending your swingarm you will need to add links to your stock chain or purchase a new oversize chain. This chart shows how many links you will need to add according to the size extension you have. Note, these are approximations. It's always best to order your chain slightly longer to compensate for any unforeseen variables.
2 inches over add 6 links
4 inches over add 12 links
6 inches over add 20 links
8 inches over add 26 links
9 inches over add 28 links
10 inches over add 32 links
12 inches over add 38 links