You should not have to change the arm. As Girard said, the center of the damper pivot must always be centered on the steering stem, so what changes is the location of the holes in the top bar clamp/mount. The only reason to change the arm length is if you change the tower design to increase the offset from the steering stem center.
I'm familiar with the checkpoint clamp, its like an Oberg Sports clamp with the locking lugs/slots. All you have to do is select a position that you like that Scotts or BRP has a top mount for. If its a problem, you can make your own. I sold my submount(too high), went back to a top mount and made a simple, custom mount plate that I use with my OEM clamp and Bike Bonz handguard mounts. This could also be used with two piece upper bar clamps.