does promotion increment get higher when you go higher grade?

Example (for illustration purposes):

Assuming MI is same (5%)

Mx 12 - Mx 11 = MI (5%) + PI (3%) = total 8%
MX 11 - Mx11a = MI (5%) + PI (5%) = total 10%
Mx 11a - Mx 10 = MI (5%) + PI (8%) = total 13%

Does PI works this way?