Many multi-meters have a section where you can measure amperage. Some go 0 to10 amps and also have a milli amp range for weak power loss. Put that between your battery and its hot connection. I think that is much better way than using voltage drop method.
Problem with this is, upon disconnecting battery to put meter inline, the fault may disappear. Typical with alternators with bad diodes.
I use a battery disconnect, install meter and open battery cable circuit
This doesent interrupt the circuit enabling a consistent diagnosis.
Voltage drop testing is ok for high draw faults and if you have a decent multi meter.
Im betting on a poor battery, from the first test at the parts store. A battery draws down to the point it wont crank,but usually recovers the voltage to something over 10. A twelve volt battery is still a twelve volts until its completely discharged. If it wasnt, a half discharged battery would be 6 volts. Not enough to turn on a light bulb.
As mentioned earlier if its more than 5 years old you are running on borrowed time.any battery that lasts over that is bonus.
You will have your battery anyway , something to compare it to the new one.