You can put a caliper or measuring tape on each end that you want the heashrink to cover. The wall plug will be the bigger one, and you'll want to oversize the heatshrink just a tad so it slips over easily. Now measure the diameter of the cord. The heatshrink will have to shrink enough to fit the cord, so if the AC plug is 2" in diamter and the cord is 1" in diameter, a 2:1 ratio would barely work, but a 3:1 would really be the way to go (note that these are not the actual dimensions of the parts).
I always recommend using heatshrink with adhesive on the inside, as it makes for a very solid connection that takes vastly more time to fail than normal heatshrink.
The heatshrink that comes with our power cord kit has adhesive lining, so it will be quite a bit more work to remove it, especially from the tech flex sleeving.
