AbleCommerce Shopping Cart Software

We're trying to set up volume discounts for products in a category.

The goal is to do this:

Purchase 2 products in category X and get a total of $20 discount
Purchase 3 products in category X and get a total of $50 discount
Purchase 4 or more products in category X and get a total of $100 discount

We have the store configured for volume discounts by category. It looks like the volume discount engine is set up on a per product basis and we can't set a maximum amount. We can get close to these discount levels if the user purchases 2, 3 or 4 products by doing this:

Purchase 2 to 2 products in category X and get a $10 each discount
Purchase 3 to 3 products in category X and get a $16.67 each discount
Purchase 4 to 4 products in category X and get a $25 each discount

The problem is this gives no discount if you purchase 5 or more products. If we change the maximum on the third tier then we continue to give discounts past $100.

Is there a way to give a cart total discount based on the number of items purchased?

Or, is there a way to limit the total discount given for the cart?

Or any other thoughts on how we can achieve our discount goal?

Thanks,