The 700 credits mark was just a figure I came up with, I didn't do actual measurements with machines and projects. Last year I set it at 600 and it went pretty well, so I upped it by 100 points this year. I just thought about setting it in a way that no single cpu machine can reach the pharmers league, even if its dual core. I guess the set value pretty much does the job for almost all situations.
There's just too many variables to be able to calculate an optimal value, for example different projects have different credits/hour rates, optimized clients work on some projects and not on others, applications optimized for specific cpu's get more credits for people with that cpu, people with overclocked cpus get more than those at stock speeds, etc...