From what you're describing, you build is Bandshoppe pole, 4# foam, blue foam, and 1/2" yoga mat.
If that's correct, then you would want to do a 4# box, 3 layers of blue foam, and then a 2-layer stab tip. Double boxing is not necessary. Using a layer of yoga mat in place of your first blade layer is one option, as Randy said, but I personally have had bad results building with that method. Your core for this size of a weapon, assuming you are using bandshoppe, which is what your description sounds like, is a bit on the stiff side. Too little flex can make weapons hit very hard, meaning that no reasonable amount of foam will make them pass hit. At 5', bandshoppe is a little iffy. It'll probably work, but you may end up needing an extra layer of foam.
Anyway, how I would recommend building this weapon, is as follows:
- Prep core. Biscuit the ends.
- 4# box
- 1 layer of foam on striking surface, on each side, covering seams (do not go up and over)
- Bevel the edge of the blade foam at the tip at a slight (<45) angle, on each side.
- 2nd layer of foam up to bevel of first, on each side. Bevel as former.
- 3rd layer of foam up and over last layer.
- Bilayered marine foam (yoga mat; assuming 1/2" thick yoga mat) for top
- Tape for stability, as shown below
- Build cover, handle, and pommel as desired.
Visual representation:
Based off of Adakan's Stable Tip Design, found
here.
Image not proportionally correct.