My method to measure time in darkness is probably simpler, although not very fancy. I recorded myself counting time every 15s from 0 to 30 minutes: 15, 30, 45, 1 minute, and so on. I start playing the record (it is stored in google drive, any device that can access works) when I turn off the lights. I chose 30min because I’m done in 23 or 24 minutes, from prewash to fix (usually, 6 8x10 sheets). The safety margin has allowed me to deal with several mistakes in the past.

Best,
Pau