In response to a discussion over on Discord...

A good start would be considering manufacturer, product lines, engine choices when coming up with a commonality number, and then displaying a commonality factor based on the 3 type rule now would work. For example, I fly 4 types (lets say 737 Classic, 737 NG, 757, and 767) that currently would have an exponential multiplier based on fleet size. Under the new rule, you have 2 737's with 80% commonality, a 757 and 767 with share 40% commonality, you use 2 engine manufacturers and one manufacturer. So maybe add things up, our target number is 3. We have 1 manufacturer, 4 types, 2 engines from 2 manufacturers. Thats a 9 from the 4 determining factors. Simple math calls that a 2.25 so there is no penalty. Add in the partial commonality and we can reduce that further. If the examples above are used, the 737's share 80% commonality, so they count as 1.2 frames. The 757 and 767 share 40% commonality, so they count as 1.8 frames. That reduces the number to an 8, so our commonality score is now a 2.

Now that is simplified greatly and there should be added weight to some numbers and reduced weight for others (number of types carries more weight than engine manufacturers, for example) but this would be a start to figure more accurate commonality numbers.