Supporter Payment Options in $ not %?

I'm confused here... I'm not exactly sure how commission payment are supposed to be set, but I am guessing that you set one $ payment for each payment period on the sign up page?

Why?

Why not make it so a percentage commission can be paid on all payments?

I'm really confused about why it has been made this way and how it can be of any value as it is now with different levels available in pro sites. If percentages could be set it would be nice, but as I see it now this can only be used if you have only one "upgrade level" or if you are willing to pay everyone on the lowest value upgrade.

Please tell me I am wrong and percentages can be set for affiliate payments.