I agree with thatguytom, Line tests are the way to go, I posted up a sample file here How do I set up a line test?. Every machine can be different so it comes down to trial and error but if somebody is getting a clean cut and can share the info, then I do a line test around that number in increments of 5 ipm to dial it in.
Also when people give feed rates, you really need to also say what machine it is, not just 30 or 45. I had a Razorweld 45 and now have the Hypertherm 45xp and feed rates are totally different. For example with the RW45, 10ga I was at 40-45 ipm for a decent cut where with the Hypertherm 45xp I am now at 130 ipt for an extremely nice cut with little dross. So far the Hypertherm cut chart in the manual has been spot on.