Nice really simple and clean and one of those, I wish I'd thought of that moments.

Couple of things I'd change.

First ( probably me being dumb ) but for some reason I clicked on "Money per month" first rather than "amount". Maybe a dollar sign in the text box would help. Although I realize this has probably been done deliberately so it's currency agnostic.

I wasn't allowed to put in period characters, I tried to put in 37.5 hours ( standard week in the UK ) and it put that as 375 hours, which screwed the results just a little bit.