Well guys, I've done some more testing and my XTi2000 starts to reach clipping light at ~23ACV, the clipping light shows clipping at full output (800), so I'm using these calculations

23/800 = 0.02875 * 500 = 14.375ACV (to get 500)

So I've hooked up a volt meter and using a Sine Wave from

http://www.binkster.net/extras.shtml I've turned up the dial on the amp to reach a maximum of 14.375ACV (this is when my inputs are all maxed out).

This should keep me safe, I usually run inputs at 75%, so I should be good because that will give me less than 14.375ACV. Until I get some further information I will use these settings.

Please let me know if I've done wrong calculations etc.