Bill,
As I recall, I only ordered one 4' x 6' sheet, and -- with very careful measuring and cutting -- I only had a few very small scraps left over. I only did the area in front of the seats, and not the "back seat" area. (Foil side down.) This is the only thing I used because the felt part of the heatshield is quite thick. I laid the carpet right on top of it. I don't think you would need an additional undercarpet material.
And, again, I didn't use the adhesive Moss sells with this material because I didn't want to glue this stuff down. That is what was done previously and I had a heck of a job getting up all the old adhesive in order to repaint, etc., etc. I used 1" wide Velcro in strategic areas to hold the material down (foil side down).
One other tip... When I installed the heatshield material up the sides of the cockpit area, I cut it about 2 inches shorter than where the carpet would rise. This left me 2 inches for velcro on the side panel and on the carpet to hold it in place. In other words, the heatshield material is velcroed into place on the sides, right and left, and in front, and the carpets are also velcroed right and left, and in front, but to the wall just above the heatshield and not to the heatshield material itself. (Sorry for this poor description). The result is that I can remove the carpet for cleaning, etc., etc., without disturbing the heatshield material, and I can remove the heatshield material for inspection, repainting, etc., etc. My aim was: don't do anything that cannot be easily undone.
Sorry, too, for being so wordy... Hope this helps.
Bill: Check my following posting for a couple of photos I just took for you.