Too many variables, in my humble opinion, to go with the "4 fingers" above the brow test. I have large hands and the fingers that go with them. As a result, my fourth finger would align so high on my scalp that I'd be paying good money to look like I was balding.

The foregoing said, I'd suggest wrinkling your forehead and looking at your reflection in a well lit mirror. You will probably be able to discern from that where your original hairline used to be (even after all these years you'll see a little indention in the shape of a hairline -- or at least I can). Adjust your hairpiece hairline according to that, going a little higher if you wish as a concession to not being a kid anymore.