I have similar concerns as you, but my current approach of copy/pasting from notepad (so I can label things) to a calculator is very prone to error or being outdated. I think the tight feedback loop in an app like this makes things discoverable and builds confidence in the output.

edit: when I'm "deep in it" this is similar to what I use Excel/GDocs for, but they're heavier than I'd like for more casual use. I spend half the time I'm using a spreadsheet in Google looking at documentation for Excel functions.