I have done it 2 ways and never owned a mainline drilling tool. My original way, I made a block of 2x4 with a hole drilled thru it in the 4" (3.5") dimension then I held my tapping bit up to the 2x4 and marked a point at that thru hole where the tapping bit just reached thru about 3/8". I marked that measurement, then cut a V in the 2x4 to center the hole over the mainline. I then cut off much of the excess wood to make it lighter and easirer to hold. To drill a hole on the mainline then I simply held the 2x4 V on the tubing and drilled. It never messed up.
My current method is that I bought a mainline drill bit from DSD. It only extends out of the chuck far enough to drill without going thru (I don't know about 1/2" my smallest mains are 3/4"). The tip is very pointed and it is made to drill plastic, it does not drift. That is what I have used the last 5 seasons.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.