The 715s woofer is able to take 3200W Peak but the Xti 4002 can "only" make 1600W per driver, so we don't need to worry about peak power (as long as you don't clip the amp which is still possible when configuring the limiter as an RMS limiter). For continuous power I would recommend limiting to about 250W per driver or even less (but I'll show you how to calculate using these numbers), that's 500W total from the amp.
So using our formula with the current numbers:
10 X Log(P1/P2)=10XLog(500/3200)= -8.02 dBfs.
That's the number you enter the amp.
If you're still having trouble, let me know...
What attack and release times would you use for average power protection?
I just bought a couple xti2's and am in the process working on the limiters.