For every step:
Multiplier (M) = (# of steps in your current combo / 4) rounded down
"Good" or worse step = 0 (and this ends your combo)
"Great" step = (M * M + 1) * 100
"Perfect" step = (M * M + 1) * 300
e.g. When you get a 259 combo, the 260th step will earn you:
-
M = (260 / 4) rounded down
-
= 65
-
= (65 x 65 + 1) x 100
= 422,600
-
= 422,600 x 3
= 1,267,800
It is not known yet how your grade is calculated.
Point of interest: The fewest number of steps it takes to "overflow" the score counter is 544 (all perfects, of course), which is worth 1,000,824,000 points. When the score reaches 1,000,000,000 points, the counter will only show 999,999,999.