How much tissue is taken off with myopia and astigmatism?

I've had PRK before and my currently the thinnest part of my cornea is about 450 microns. My eyes now have the following prescription SPH:-1.50 CYL:-1.75 I understand that for about 1 diopter 15 micros of tissue is removed. How does astigmatism come into play here? Since I'm -1.75 astigmatic, does that mean that about 48 microns will be removed? 1.50+1.75=3.25 3.25*15 = 48