I don't know what the equation is. I know that there are on-line calculators that try to tell you but I haven't spent enough time to determine whether they can be trusted.
But to answer your question, if you are satisfied with 73 to 85dB in your 3,000 cubic foot room at 8 feet away, then a 90W/channel amp should do you just fine. You're planning on 60s vs. my 80s (which are more sensitive) but based on recent testing that jakewash and I did, the sensitivity of the 60s is pretty darned close to the 80s.
