Holy crap, going to school for engineering is actually going to be useful for a change...
The formula for calculating internal resistance of a battery is;
Rb = (Vs - V / I) - Rl
Where .. Rb is the IR of the battery, Vs is batt voltage w/o load, V is voltage under load, I is the current supplied by the battery, and Rl is your total resistance in the circuit.
In theory if you have a known load source that is carbon pile or resistor based you can know what our circuit resistance is.
However unless you've got good testing equipment or are willing to build a decent load source your best efforts will probably be +/- 0.1-0.2, which almost defeats the purpose.
I would not worry about matching IR. The packs your getting have already had their "goodness" worn off already and are a little soft anyway, worrying about IR is a waste of time. Just do what PitCrew said, and what you are already doing ... that will get you close enough and you can have some decent club race packs that way..its how I got through my first year of racing myself before I realized I wanted/needed *really* good packs and started buying new.