Calculation of order total

Hi -

I am on OC 1.5.1.3

I am using a Sales Promotion extension and am using Coupons. I have my order totals sorted: Sub-total, Sales Promotion, Coupon, Shipping, Taxes, Total.

For the purposes of this example, let's assume the Sales Promotion is 10% off and the coupon is 20% off.

My site is working as follows:
Subtotal = $100
Sales Promotion (10% off) = -$10
Coupon (20% off) = -$20

The Coupon is applied to the subtotal, ignoring the calculation of the Sales Promotion.

Is it possible for the Coupon to be applied to the remainder (after Sales Promotion is applied)?
Subtotal = $100
Sales Promotion (10% off) = -$10
Coupon (20% off) = -$18 (20% of $90)

I guess I need to better understand how the sorting of the order totals works. Am I missing something?

Thanks in advance.

I thought I was asking an easy question... Any input?