Commission Calculator

I am looking for some help with calculation commission for my business.

I have a Form called Policies and a form called Renewals

For each policy, there will be 1 renewal each year, and commissions are paid on the total premium of the renewal.

However, there are 2 different types of commission. a flat % or a "graded scale"

I have sorted out calculating the flat %, but I need to be able to figure out how to calculate the graded scale.

The graded scale works like this:

10% of the first $10,000

7.5% of the next $15,000

5% of the next $25,000

3% of the next $50,000

1.5% of the next $100,000

1% of the remainder

Here is a Table with 2 example premiums. one of $300,000 premium and one of $60,000 premium

		PREMIUM:	$300,000	$60,000
Scale 1	10%	$10,000	$1,000	$1,000
Scale 2	7.5%	$15,000	$1,125	$1,125
Scale 3	5%	$25,000	$1,250	$1,250
Scale 4	3%	$50,000	$1,500	$300
Scale 5	1.5%	$100,000	$1,500
Scale 6	1%	Remainder	$1,000
		TOTAL	$7,375	$3,675

To make things even more complicated, there are multiple different scales (only one scale is used per policy) so i need to be able to select with scale to use in the policy record, and then have all related renewal records use that scale to calculate the commission based on the premium of that renewal

I hope this all makes sense