When tapping a pipe plug hole, it is important to stop and try the plug in the hole often. You can always go deeper with the tap if needed, putting metal back in the hole is the hard part !
Cast iron will cut very easy without any cutting fluid. Drill to the smaller side of what the chart says incase your bit wobbles or wonders. Tap slow and steady. The plug should almost be flush when finished, if the cast iron is thin where the hole is, finish up with the plug sticking out of both sides. Just don't go to deep.
