Always easier to use a ruler. Knowing the BBTG on each iron and doing it that way can work, but in my opinion that’s doing it the hard way. Also, depending on the shaft you use, there may be tip trimming for flex (parallel tip), then the butt trimming, of course, to length after installation. Knowing the BBTG measurement and subtracting it from the length after the tip has been trimmed (parallel tip) or even just prepped (taper tip or discrete length parallel tip) would tell you how much to trim from the butt to get the final playing length, but honestly it is much easier to tip cut and or prep, install, measure, then cut to the desired playing length. This will account for any variations in hosel depth, sole radius transitions from club to club, etc. My professional advice, use a 48″ ruler or some specifically designed club measuring tool.