Working out an unknown fixture channel-by-channel and scribbling it into a spreadsheet works, but it's easy to lose track of which value did what. Enter how many channels you think the fixture has, generate a row for each one, and fill them in as you test — dropdown for the common DMX functions, a notes box for anything specific, and a value range field for things like "0–127 = closed, 128–255 = open." Export the finished map as a PDF once you're done.