I used to do tool and die/machinist work at an earlier job. Obviously there are tolerances for everything. Even seasoned cast iron takes some care to end up with a flat surface. By flat I'm talking as close to perfect as we could get it. under .001 and as I recall around .0005. It usually took a couple attempts to get here. This wasn't production but tooling related so we weren't proficient at it. We used a Blanchard grinder. What I do recall is it's extremely easy even under a literal flood of coolant to have the wheel lose just enough of the sharp exposed grain to have the piece start to heat up. This caused the cast iron to raise towards the wheel compounding the issue. This can happen amazingly fast. You are constantly watching what's happening in case you have to suddenly crank the wheel to raise the head. So we usually stopped several times to dress the grinding wheel. One last time just before the finish grind. Just kissing the surface on the last grind. What I'm saying has already been said. How much more are you willing to pay for that small gain in flatness.