Promotion - item specific, but limit total order discount

Trying to set up a promotion for a particular product line and we want to limit the total discount per order. For example, we'll discount 2 expensive items, or 5 less expensive items, but the max discount you can get will be capped at $500. I see where item-based promotions has a "discount up to" field, which reads as if it will allow the discounts on an unlimited lines until the total discount on the order hits the set amount, but it doesn't work this way in testing. Instead it's limiting each line to the max discount. Support has had