You don't need a computer. You need a calculator, and a calculator function is already built into computers. So getting a special "scale calculator" is completely unnecessary, as any calculator, either a hand-held one or one built into your computer or laptop or tablet, can "do the math."
If you like the idea of using a special "scale calculator," that's fine. I'm just saying it's not needed, as any calculator can be used to figure out scale conversions, inches to mm or cm, etc. The reason I brought that up is because the OP mentions that Apple users are "left out" because his scale calculator doesn't have an Apple version... but the fact is a special scale calculator is unnecessary. If your device has a built-in calculator function, you already have all you need to do any scale conversions.