I saw several apps - one a knee goniometer and another a goniometer. The knee one was more expensive ($4.99) and had poorer reviews. The other ($3.99) by Jinfra had rave reviews and can be used anywhere - maybe even as a level for you handy carpenters. There's a third - unreviewed for $.99 called simple goniometer that looks same as rave review one.
I don't start PT for two weeks, so haven't tried it yet.