How to calculate tier split percentages in excel

How to calculate tier split percentages in excel

I need to find a way, in excel, to apply the correct percentage to a
calculation having the following scenario:
The is a current monthly volume named month_volume and also an
accrued_amount which is the total volume on any giving month. The
percentage to calculate commission amount is constrained by the following
clause:
accrued_amount is between 1-200 --> monthly commission percentage is 5%
accrued_amount is between 201-800 --> monthly commission percentage is 4%
accrued_amount is between 801-99999 --> monthly commission percentage is 2.5%
I simple VLOOKUP function will allow me to find the monthly commission
percentage testing the accrued_amount on any giving time, But the problem
is on months that the accrued amount pass from one tier to the other. I
need to slip the month_volume and apply different percentages. The example
bellow show the situation of having a combine commission percentage on
September:
August:
month_volume = $150
accrued_amount = $150
commission = 5%*$150
September:
month_volume = $150
accrued_amount = $300
commission = 5%*$50 + 4%*$100
So I meed more than a VLOOKUP for those months that the accrued_amount
pass from one tier to the other , Anyone can point me on how to accomplish
that?
Thanks