Thanks for the reports! Everything on the back-end is secretly a float, so rounding should preserve fractional values fairly. That being said I’ve no idea why a -1 would go from 10 to 8 unless you hit an extremely pathologically rare floating point rounding edge case. Or there’s just some other bug.
I don’t expect to take the concept any further in its current form, though I do like the simplification of magic systems into a press your luck / unresistable damage option instead of a rote litany of Fire 1, Fire 2, Ice 3, etc.