I’ve done it successfully many times on the gen1 tundra just by doing it by volume with the car cold at 80F. Whatever comes out is 80F, whatever went back in was the same. I did it 3 or 4 times on that truck. I’ll bet I wasn’t perfectly accurate. Also added a spin-on ATF filter and associated plumbing. Guessed at the additional volume. Drove for years, with heavy towing. Was too scared to try the little plug on the bottom.
Bought the lexus (well used) and thought I might as well learn. Screwed it up the first time and obviously didn’t put enough back in. It’s not that the overflow straw stops draining, it’s more that the overflow straw slows draining to just the right stream. Goodness how imprecise is that? And that trans starts to warm pretty quickly during that procedure, as it drains, while the engine is warming it up with hot coolant. So the first time I tried the straw, i waited until it was just a drip. When that slipped and you could hear gurgling on hard take-off, I refilled it and drained it and then capped when when the bulk of the draining seemed to slow and it was more of a casually thin stream.” its been fine since then.
Doing similar on the F150 after the dealer couldn’t (wouldn’t) do anything else to investigate drops to Neutral when missing 3rd gear up shifts, I finally decided to swap some fluid. It was a couple of quarts low from the factory. (That didn’t fix it, but the observation was noted).
I’m not entirely sure these things are nearly as picky as DIY Middle Earth makes them out to be.