Id say equivalent to 8 maybe even 10 inches. Its not the height that gets ya it will be the width. Ive got 38.5x15x15's on my 91 ford f150 (whose fenders are more favorable to larger tires) I had to put 8 inches under it and do some front fender reforming. You will only be 2.5 inches smaller which is only 1.25 inches further away from the fenders. You may also have to do some front bumper customizing or go bigger lift, I dont know of anyone getting 8 or more inches without having to make a custom lift. I think with 6 inches you can only go with 36 or 37 12.50's, your putting 3 more inches out on each tire (thats a lot). Also make sure you look at the true tread width, it may actually only be 13 inches wide at the top all tires vary. Hope this helps.
I know SmokenMini has (I think) 36x13.50-20's on a 20x9 or 10 inch wheel. I think he had to do the pinch weld mod. He has the ProComp coilover and a PRG spacer for around 8"+ of lift. Might PM him for info?