A testing method that has worked for me is somewhat similar to what you have done. You should be able to do it with one roll of film.
With the lens cap on run off a couple of exposures. Then take the +/- exposures of an 18% gray card filling the entire frame (don't worry about focusing). Develop the film according to mfg's time/temp. When fully processed, wash and dry take the leader part that you did not expose and at the enlarger height for an 8x10 enlargement and f8 make a test print a couple of seconds each as you move the card. Process that print and when dry see where you can tell a difference from pure black. Cut pieces of printing paper and label with pencil on the back. Print each negative at the time you determined a change in the black. When processed and dry find the one that is closest to 18% gray and use that for your personal ISO. Shoot various subjects at that ISO. When you print you can see if some tweaking with contrast etc is needed. When shooting you can also tweak if necessary but this is a good standardizing technique.
IMO it's best to keep certain factors constant and make changes to fit a particular situation.