Short answer: I brute-force every single possible attack vs. defense scenario in the game. Then each one of them gets weighted by how likely it is to occur. Range, base dice, action economy to modify dice. I.e. 75% chance to have focus on offense and 50% on defense, range 1-3 bins at something like 25% / 50% / 25%, and base # of attack and defense dice about the same as the Regionals / Store Championship results. So no, it's not a time-variant formula.
How did you come up with that weighting? Do you have objective data for things like how often a focus is available, or is that just your personal opinion of what the weighting should be? This is a rather important question given how often your numbers are treated as absolute fact instead of just your own analysis.