distance from RHP to perch?

[pdomensis]

I've finally begun work on my permanent enclosure for my Bredl's, 4' high and 39" wide by 2' deep. I'm starting to work on the insides now; background, vents, etc and I'm about to mount the climbing branch/basking spot. I don't have my RHP yet so I'm looking for some advice on ballpark distances from the RHP to the basking spot so I get the perch in the right place. It will be an 80 watt, haven't decided on the make yet. I'm assuming anywhere around 12 to 18 inches would be good, but wanted to check before I do anything too permanent. Thanks!