It's easy:

Two ways, depending on what you have available to you -

1. Time: take 10% of the HAT - that will give you time (in seconds) from VDP to the end of the runway (or MAP). If your HAT is 300, 10% of 300=30, therefore you take 30 seconds off of your time from the FAF.

2. DME: divide your HAT by 300. If your HAT is 330', 330/300=1.1. Your VDP is 1.1 miles from the MAP. Back off 1.1 DME from your MAP, and you're good to go.

Hope I didn't screw that explanation up...