Using Promotions That Won’t Lower Reference Price

Does anyone have a good method for using promotions in a way that avoids lowering the reference price?
Amazon’s policy is fairly ambiguous in how it actually works. It mentions “limited time” discounts shouldn’t affect reference price but it doesn’t define “limited..”
We tried a couple of $1 coupons but some of them wouldn’t process because they had to be lower to meet the 5% minimum off limit for coupons. The $1 WAS 5% lower than the list price.
We manufacture the products and are the only seller.
We offer a ‘Buy $39 Get 15% Off’ promotion regularly and aren’t sure how this affects individual ASIN prices.