Imperial standards are dinosaurs, sprocket. (And for good reason!). They are effectively dead. Better stop torturing yourself and get with the decimal system already. You may not like it, but it is current reality and the foreseeable future. You're fighting the inevitable. That's pointless. And a waste of effort you could apply otherwise.
As to your question: I would think the best free app I can think of is either a note app, or a pencil and a piece of paper.