process early bird discount before custom fields

i have an event for $158 regular price. the Early bird takes 29% off. I also have a custom field for $25 off.

The problem is that the early bird discount is taking the 29% off after the the custom field is taken off first. so instead of taking 29% off $158 it is taking 29% off $133.

the formula should look like this
($158 - 29%) - $25

So how do i get the early bird to discount the regular price without subtracting the custom field first

In this case, I think the easiest way is that you set the discount to a fixed amount instead of by percent !

Tuan