Whether you think of it as a guess, that's exactly how Excel does it. It plugs a number (possibly zero) into some cell in the cycle, and then computes everything (including that cell) based on that assumption. The newly computed value is the next guess. It repeats until the value converges, if it does.
Break the cycle. To do that, copy the formula from your "answer" cell into some new cell (which I'll call the "next guess" cell) and remove the formula from the "answer" cell. Nothing else in the spreadsheet needs to change. Convergence means the value in the "next guess" cell, as calculated by all the formulas in your spreadsheet, matches the value in the "answer" cell.
I showed you an AppleScript that would iterate once, but it could be easily expanded to iterate 10,000 times if that's what you want. On each iteration, it compares the values in the "answer" and "next guess" cells. If they're close enough, it quits. Otherwise it copies the "next guess" value into the "answer" cell, which triggers Numbers to iterate one more time. Count the iterations (or look at the elapsed wall clock time) and abort if convergence is taking too long.
