I bring two pieces of bubble foil insulation, a small one as a seat pad and one that covers my sleeping pad. When it's warmer out I'll use the bigger piece under my NeoAir like a ground sheet to protect it, when it's cold I bring a piece of plastic visqueen as a ground sheet.

So, to answer your questions, the R-value of bubble foil is, as finallyMe pointed out, 3.2, and you can add a value of 1 to that for every inch of airspace you seal off with it. On a 3.5" thick wall that would be a total R-value of 6.7.

You want the shiny side up so that it reflects your body heat back at you, but it will still do that no matter which way you put it. Some bubble foil insulation has two shiny sides.

For your purposes, if the NeoAir keeps you warm enough as it is, you can bring one small piece and use it for both a sitting pad and to extend your 3/4 pad to full length insulation and I think that's a great idea.