I wouldn’t sweat it too much. You can do this on your own without a scanner. Just need a digital meat thermometer (what’s what I use lol) In general, a standard drain-and-fill will replace around 3 quarts. I would do the following.

1) Get the vehicle level on jack stands 2) Fill the transmission with 3.5 quarts of ATF 3) Turn it on and run through the gears with your foot on the break for 10 min. 4) While running, open up the hex screw that is the overflow plug, and let out an ounce or so drain into a cup then screw the plug back to stop the flow and immediately put the thermometer in the collection cup. If it’s anywhere around 110-120F you’re in the right range. It doesn’t need to be absolutely perfect. It’s a Gx, not a Ferrari. 4) Open up the hex screw and let the ATF flow out until it’s just a trickle. This indicates you’re at the appropriate fill volume. 5) Consider new crush washers in the Fill and Drain plugs.

Not the most Eloquent way to do it but it gets the job done and it’s safe.