Good Morning Capt.
That's a good question, and yes, you are correct that every manufacturer has their own little way of measuring what is standard for length. At Mizuno we use what we call a "UN Gauge." This takes lie angle into account (as many of the length gauges out there use only a 45 degree board). What we do is we set our UN Gauge to the proper lie angle, and then we measure our length to the line just below then end of the grip butt cap (this is the line at the base of the chamfer at the end of the grip where the diameter is the largest). I hope that makes sense and let me know if it doesn't and I'll explain further....
-chris