Use the covered end of the cap and don't worry about the radius, it will grind to what the mirror is after a while. You can use the ring end if you want but you will need to drill a hole into the side or top to relieve air pressure inside the cap. a 2" cap is a nice size for an 8" mirror. You spend most of your grinding time in the middle with lesser amounts towards the edge. Semirandom strokes are the way with this one with the preponderance of the time spent in the middle. A straight edge (or a curved piece of cardboard or other material) will show where the surface will want to be carved mostly although you will find that the curve generated will be a fairly decent spherical surface anyway. The basic process just makes spherical surfaces anyway. The object is to get most of the material out of the hole before you start with the tile tool. Several of the guys in the telescope making group have done it this way and they have all inherently done it right. You do need to do some grinding with the coarse grit after making the tool but that's mostly to get the tiles in near full contact with the mirror. Don't worry about the plaster as it will erode right along with the tiles (it's a bit softer) and the only problem that could be had is bubbles in the plaster which can easily be filled with some epoxy.