Need help making an exact size marble "ingot"
Marble is easy - this is not at all dissimilar to finely fitting marble tiles, and marble being a relatively soft rock, you can wet-sand it, or use a silicon-carbide block from the tile section at the home improvement store to dress it to precise size. The block will last longer, but is quite coarse - wet/dry sandpaper can be had in a number of different grades from very coarse to very fine.

Without knowing for sure, I also wonder if you are merely up against a "budget tile saw" or if you are forcing the cuts rather than letting the saw cut at a speed it's happy with. Most diamond-blade tile saws (particularly wet ones) make quite good cuts, when run properly, unless the whole thing is really shoddy.