>With ANY decimal numbers you will have some rounding errors. Sometimes the programs masks the error, and you get the result you want, and sometimes you get the wrong value, there's simply no way to avoid that. You can calculate with a million decimals and minimize the error, but you simply can not eliminate it. Everyone knows that (1/3)*3=1, but still all the calculators and programs I have seen shows 0.99999. In practice it's the same problem, it's only easier to see it.

Fox seems to have some internal tricks in place, probably rounding to the 15th significant digit... I've tried

(1/3+1/3+1/3-1)*1e46

with the last number varying between 1e9 and 1e46 arbitrarily, and the result was consistently 0.00, even when I set decimals to 12.

My favorite trick with fixed (non-float) calculators was to take square root of 8 and square that. The difference was amazing - sometimes I'd get two last digits different from 9. Of course, never got an eight, not in fixed-point.