Get a vernier gauge, measure the thickness at thespine across three or four points, and then measure the height from the spine wear strip to the cutting edge and calculate. If the bevel could stand to have the edge reeled on and the spine kept as is, use tape, if the spine is already creating an obtuse bevel don't use tape for now... If Inhave to remove big chips on wedges or grind through pits to hit a bevel I usually calculate, see where I am and then go without for a time before defaulting back to one layer. I wouldn't personally go over two layers as tape is compressible. Onlu exception I make are for 18th century stubtails where the spine tends to be really thin.