SuperStar YG

Skill Multiplier:

Super Perfect - 2

Perfect - 1

Good - 0 

Miss - -10

Calculation of Music Battle Score after Skill Multiplier:

Assuming that A is Player 1's Skill Multiplier, B is Player 2's Skill Multiplier,

Bonus Multiplier = (1 - (B/A)) x 2 + 1

Final Score = Raw Score * ((1 - (B/A)) x 2 + 1)

If the Bonus Multiplier is below 1, the Final Score is the Raw Score.

Bonus Multiplier is now capped at 3x after 210604 Music Battle 2nd Window.

Note that the values of A/B can go into negative, despite the values being displayed as 0.

Case A: Player 1 Battles Player 2 on Island, with a total note value of 446. Player 1 has a R1 deck, and gets 446 SPs, while Player 2 gets a score of 421 SPs, 18 Perfects, 7 Goods and 0 Misses.

Player 1's Skill Multiplier = 2 x 446 + 1 x 0 + (-10 x 0) = 892

Player 2's Skill Multiplier = 2 x 421 + 1 x 18 + (-10 x 0) = 860

Bonus Multiplier = (1 - (860/892)) x 2 + 1 = (1 - 0.9641) x 2 + 1 = ~1.0717

Final Score = 2,595,000 x 1.0717 = 2,781,188

Case B: Player 1 battles Player 2 on High High, with a total note value of 532. Player 1 gets 532 SPs, Player 2 gets 532 Misses.

Player 1's Skill Multiplier = 2 x 532 + 1 x 0 + (-10 x 0) = 1064

Player 2's Skill Multiplier = 2 x 0 + 1 x 0 + (-10 x 0) = -5320

Bonus Multiplier =  (1 - (-5320/1064)) x 2 + 1 = (1 + 5) x 2 + 1 = 13

However, as the multiplier now has a cap, the Bonus Multiplier is now capped at 3x

Final Score = 60,000 x 3 = 180,000

Credit to @ryanlulz, @jQ for the mathematics, @Jazz, @Cane for the source data.