Large Drum 71.323/1.296 = 54.96 Revolutions.
Small Drum 19.202/.753 = 25.52 Revolutions.

I'd use multiple magnets.
It may be easier to just experiment.
Plug in a number and see what you get for results.
If for example, your meter only reads 35% of your measured amount, divide the calibration number used by .35 and try again.
You may find you have to use more magnets on the "rim" of the drum to get enough resolution/accuracy.