Are you sure the gauge block fixed your offsets and not just your method of setting them? The feeler gauge method has you guessing at what "drag" feels right, and that changes part to part, especially if the holder or spindle nose has a burr. A 1 inch block gives you a hard stop every time, so of course your numbers repeat. The taper probably wasn't the vice at all, it was you touching off a few thou different on each tool and the error stacking up over the run.

You really want to trust a tap code when your life is on the line at 60 feet? Man, I get that you two had a system and it worked, but comms is your lifeline down there and a dead mic means you lost your surface team's eyes and ears. What happens if you get snagged on something and can't reach the pipe to tap on it? I've seen guys try hand signals in murky water and the tender misread the whole thing. If my box quits, I'm surfacing, simple as that. No wrench pattern is worth betting your air supply on.