Whatever their version of a cubit was, they could cut a string that would be, for their purposes, one cubit. Got that? Having measured 10 cubits for the diameter, how difficult would it be to measure either 30 (which they did) or the more accurate 31 (plus a tad) of them for the circumfrence? Too high-tech for those guys? Really?
Don't get testy with me Patrick.
Why don't you go search the Bible and find the verse which refers to tenths of a cubit. Neither you nor I know what they measured or knew but one of us is pretending he knows. That would be you. Got it? :>}