Hi Jarrod,
The conventional method is to use 2 glass disks. Grind them with the hogging stroke and one surface goes concave (hitherto known as the mirror) and the other goes convex (hitherto known as the tool).
You can substitute one of the glass disks with a tile tool, as you have done.
However, it's more efficient to use a sub diameter steel tool to hog the mirror, because the steel is softer than the glass and the abrasive chip momentarily digs into and is held by the steel to abrade the glass more efficiently than just rolling between two surfaces of equal hardness.
The tool doesn't have to be anything special. I use some 3mm thick steel that is glued to some plywood. See photo. For an 8" mirror, a 4-5" tool would be OK.
Once you have reached your depth (sagitta), you THEN cast your plaster tool using the hogged mirror as the mould to get a convex tool. see photo. Once cured, you coat the plaster with any old paint or varnish you have around the place and glue on your tiles using epoxy. BTW, the red plastic mould is from a plastic plant pot.
The advantage of casting the plaster on the mirror is that the plaster will then have a convex surface and so the tiles will wear evenly during smoothing and fine grinding. BTW, I use glass tiles.
Once you have finished with fine grinding, cast another tool to make your polishing tool.
Good luck
