I ask because, using the 100gh x 4, if we set one worker to 286, thats what each unit would try to run which seems to high where as using a diff of 71. That would seem more appropriat for a machine doing 100gh.
The mining server looks at the hashrate per worker. So if you run a lot of hardware as the same worker but set a minimum difficulty based on the hashrate of just one of those devices, then the mining server will probably set a higher difficulty for you.
Anyway, the mining server will mostly take care of this for you. Setting a minimum diff is most useful for the first few minutes of mining so the mining server doesn't have to ramp you up from diff 4 to diff 512 in a hurry, it can start on a difficulty that makes more sense for your worker.