To establish that the battery has the capacity claimed , discharging down to BMS cut off would be my method, one deep discharge will have little effect on battery life.
If you feel this is too severe then using a combination of battery voltage and the battery SOC may give you an idea of capacity.
A resting battery, no load or charge, will settle to 12.0 volts at around10% capacity.
You don't need a high current discharge, a 5 amp or 10 amp load would require a longer test time, ( need not be a continious test period).
Just use whatever is convenient and makes the sums easy. A more accurate and low cost method would be to use a Dork load monitor with a hall effect sensor, ( shows a UK source but available from Ebay or Amazon).
With the Dork it just measures power for any load current.
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Other points, your 100Ah battery is limited to 100 A continuous discharge and 50 amps charge
When you are testing the battery at high loads, review the cell volts. They should not have a significant voltage difference between each cell over the 20% to 90% state of charge range.
Mike