Was daydreaming today, be surprised if no one has done this before. I did a brief search but saw nothing.
I found a gallon jug and timed how long it took to fill. 8 seconds. At first I was going for 50 gallons but my tub wasn't big enough. So I halved everything to 25 gallons.
8 x 25 / 60 = 3.33 minutes or 3:20 to get 25 gallons in the tub.
Pool math said add 0.3 oz of 12.5 bleach for FC of 10, 11, 12, and 13 so I figured 12 was close enough to the middle to avoid rounding error. Using the TFT vials I eyeballed 8.8 ml (.3oz) of bleach, did the test and got 22 drops (11 FC)
It's not the most scientific in the world, but it's close enough for a sanity check. Maybe someone will find this useful.