Just curious why when using a discount code you aren't allowed to make the discount as a percentage of the rent? You can only make it a percentage of the subtotal or a flat rate.
This is an artifact of the way rate data from listing channels is stored. It's a known issue and there are plans to refactor this area and add this capability among several other related hot topics, but that's a good bit of work.
In the meantime, this workaround may also be helpful: