Optimal SM tanks bigger than 100s

[tbone1004]

I'm about your height, and I find LP85s to be the right tank for the job. I tried 95s, but they felt almost "too short" especially for how fat they are. I tried LP 108s once, too. Those are not too short, but they are still 8" fat so still annoying and more volume than I feel like I really need. Honestly, I feel the same about these tank sizes for back mount too, but for slightly different reasons. In either configuration, the 85s are more streamline, seem to be the right length to trim out well and not hit me in the wrong spots, and hold almost the same amount of gas as the fatter tanks.
If your only concern is gas volume, however, here's a quick comparison of how much each tank you are considering (and a few more) holds at 3600psi
HP100 - 104cf
HP120 - 125cf
Steel72 - 115cf (if you dare to overfill by 60% over rated working pressure)
LP85 - 116cf
LP95 - 129cf
LP108 - 147cf
LP120 - 164cf

You forgot Z-Factor

Within Faber
LP85=FX100
LP95=FX117
LP108=FX133
LP120=FX149.

That Z-Factor is very important.... It will get you killed on a rebreather if you aren't careful you'll come up 10% short. On OC it doesn't matter since the smallest third is the one you breathe on the way in, but if you're on a rebreather doing bailout planning it must be factored in.

 

[grantctobin]

If your only concern is gas volume, however, here's a quick comparison of how much each tank you are considering (and a few more) holds at 3600psi
HP100 - 104cf
HP120 - 125cf
Steel72 - 115cf (if you dare to overfill by 60% over rated working pressure)
LP85 - 116cf
LP95 - 129cf
LP108 - 147cf
LP120 - 164cf

Your math or your understanding of gas laws need quite a bit of work…