What if I have 4 different rigs, totalling 2.7Mhs, but one has 400MH, the other 700MH, etc, all pointing to the same BTC address? Should I just leave "x" in this case?
Overall you're better off setting a hardcoded difficulty. Lowers the work on the server overall, and if/when you disconnect, you don't start back from the default difficulty and ramp up. I'd personally use 64 for the 400MH, 128 for the 700MH, and 256 for anyhting around 1MH (512 above 1.5ish?). Theres no right answer, they'll all work fine, just a bit differently (more/less variance, more/less network connectivity, etc)