Here is a simple example:
Property advertised price: $500,000
Purchase price: $450,000
Discount: $50,000
My commission can be fixed 5-10k depending on the price of the property and will be deducted from the discount. However if I can only get a discount of $4,000 then that will be my commission. If I get no discount at all then my commission will only be the engagement fee which is $1000.
The other option is to use the % method. I get 30% of the discount I can negotiate and the buyer get's 70% of the saving. In the example above that would be $15,000.
The option we chose depends on the price range and the option the buyer wants. We decide this before we start looking at properties.
This method aligns both our interests. Most buyer agents will just flog you anything since they are getting paid either way. They just have to buy you a property.
Selling agent and seller interest are both aligned but I feel that buyers are not properly represented by buyer agents due to their fee structure.
I see what you are trying to do, but the advertised price may be inflated and I don't believe it is really a valid benchmark for the purposes of calculating a fair discount, if the buyer chooses to use the % method.