I've been working on the Fusion formula for this game:
Card3 = BaseCard x LVLConstant + Card1 x BoostConstant + Card2 x BoostConstant + Card1MasteryBonus(if card1 mastery is maxed)
Card1 and Card2 represent the Attack or Defense value.
The LVLConstant for the 1st fusion is 1,1, for the 2nd fusion is 1,33 and for the 3rd fusion is 1,606. The BaseCard is always the most basic card.
The BoostConstant is 5% if the card isn't maximum level and 10% if it is.
Card1MasteryBonus is only added if the mastery of Card1 is maxed.
Things missing: How is mastery bonus calculated? Mastery Bonus seems to be 37,47% of lvl1 Card. For all fusions using Card+ or Card++ and so on and so on. I tested this for multiple cards of different rarity and there are several exceptions so we can only estimate the bonus. The value shifted around 0,1% of the Base Lvl1 Card value. Conclusion 37,37%-37,57%
What's the constant that represents the power of a perfectly mastered 8-15 fusion (in a 4fuse card like Black Widow) and in a perfectly mastered 2-3 fusion?
I have few specimens to test this but: Perfect 2-3 fusion = Maxed Card1(lvl and mastery) x 1.3999