As stated previously, with independent doubles I would route the right SPG along the shoulder strap and it worked well.I don't know about that. I've clipped off tanks and the bolts run down and to the left. The spg would be up and to the right. I also wonder what would be worse, managing the clips on the upper D ring where you can see then or on the hip where you cannot. I'm not even sure the spg needs to be clipped to the chest D ring. I can see it being routed beside the LP hose and secured with zip ties (or cave line if one prefers).
What's simpler. A spg routed beside an already existing hose that can be read without unclipping or adding a 30" hose down the left side of the body that needs to be unclipped in order to be read.
In backmount I carry the SPG on the right hip about 85% of the time. The other 15% I tend to carry it clipped to the left chest D-ring - specifically when carrying multiple stage/deco bottles or when carrying a stage at depth or in fairly tight tunnels. While I can fish an SPG out from under a couple stages, or in a tight tunnel, etc, I see no reason to actually do so. It makes sense to keep the SPG on the upper D-ring under some circumstances and having it there also tends to lead to its more frequent use.
Now, the counter argument that you'll get form the DIR crowd is that moving it is not needed and that any suggestion that it is "easier" is just using an equipment solution for a skills deficiency, regardless of whether an actual skills deficiency exists. So in effect, the implication is that you should do the fish it out from under the deco bottle thing because it is harder and consequently doing it will prove you can do it. That is PFS.
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It is true that you should be able to predict what the SPG will read and that is important as any deviation from what is anticipated suggests a problem (leak, roll off, etc, or at least a deviation from the potential plan (slightly higher than expected SAC, etc).
However, too many people take it to the extreme and a subset of those seem to take pride in a doing an absolutely minimal number of SPG checks per dive. I my opinon, that just reduces your SA by eliminating one potential source of information - which is again PFS.
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As for potential confusion between the back gas SPG and the stage SPG(s), they rotate on the end of the hose. If you cannot tell the difference between one near or clipped on the D-ring and one mounted on a stage bottle clipped to the same D-ring, you have serious issues, but even then you can just turn the not in use SPG(s) to face away from you - which will eliminate the confusion - even if you are PFS.