I've just done my annual battery testing and my process is similar.

I happened to automate the process and as such it works like this. I
have a small PLC that does the work. The battery is connected to a
resistive load to generate a roughly 0.6 Amp draw. The PLC looks at
the battery voltage once a minute until it reaches 10.5 volts ( I
chose 10.5 because I did want to stress the battery a little). The
PLC records the total number of minutes and once the 10.5 volt limit
is reached the load is disconnected and the battery is switched over
to a charger for a recharge. I can start a "test" at any time and
come back later and read the run time. With the battery going back on
charge automatically, I'm not worried about it sitting discharged at
the end of the test.

You are going the right way with this sort of a test. The meaningful
test is to check whether the battery is still performing as the maker
intended. Apply a load which will discharge it in 20 hours and find out
how long it takes for the voltage to drop to 10.5 volts (1.75v per cell).
The 20 hours and the 10.5 volts are the standard conditions used by
battery makers when claiming their capacities.

Having an automated tester will save you the boredom of watching a
voltmeter!

If you discharge a battery faster than 20 hours, on test or in the air,
you won't get the nominal ampere-hours out of the battery. If your
electronics (especially some radios) don't work down to 10.5 volts then
don't count on the endurance predicted from ampere-hours divided by load
- and if you fly at low ambients you will lose yet more capacity.

Some battery makers websites, eg Yuasa, have a lot of useful information
on how batteries behave with varying loads, at low temperatures, and when
subjected to repeated charge and discharge cycles.