In my continued attempt to account for the variance in calculated stats, I've been running the numbers with different approaches. While I'm no closer to cracking the code, I can tell you one thing - the variances average out when you combine the stats of several cards.
While not precise, there comes a time that it helps to know the approximate max level of a card and if you should invest in it. Let's take [Adamantium Claw] Wolverine, which does not have max stats posted. Assuming that for the "+" version of the card that the card will be max level and max mastery (because why wouldn't you?)... the math is as follows:
BASE: 1.000 Wolverine: 3220 2800 MLB: 1.500 MLB: 4828 4199 MAST: 0.375 (MLB*0.25) MAST: 1208 1050 MAX: 2.875 (SUM) MAX: 9256 8049 BASE+: 1.775 (Base*1.2)+(MAX*0.2) Wolverine+: 5716 4970 MLB+: 1.800 (MLB*1.2) MLB+: 5795 5039 MAST+: 0.450 (MAST*1.2) MAST+: 1448 1260 MAX+: 4.025 (SUM) MAX+: 12959 11269
Without variance, the max ATK would be 12,959. Knowing the actual max stats and mastery bonus for the base card lets me estimate that max ATK will actually be closer to 12,977. While not perfect, you'll be able to know in an instant how he ranks against [Friendly Neighbor] Spider-Man.
Additional note: if you don't fully master and level both base cards on an important card, you're doing it wrong. The calculation for fusing two unleveled base cards then maxing yields a card with stats only 3.550*BASE - in other words you'll be missing out on about 1500 ATK.
Add the ATK ad DEF stats for SRare Hulk and SSRare Ghost Rider, and total Max Level Bonus equals almost precisely 150% of the total Base and the total Mastery bonus equals almost precisely 37.5% of the total Base. In fact, the stats add up perfectly for all parts of the equations above. Which is odd since the Hulk DEF stats skew pretty far from expectation. My last hypothesis was that the base stats were derived from the max stats somehow... but that's total bunk. My next hypothesis is that all cards work off of a total base with a "balancing" variance applied. The more a card varies from average, the more the calculations will deviate. I still need more data... and more ideas.