Back when this stuff was new, the hvac side of the equation was still being treated as an afterthought/aftermarket kinda deal. But all the other oems did the same unless the system being addressed was a true integrated hvac unit.
The various ihc service manuals do have some extensive information regarding "some" parts of these systems, but in other cases there are some major failings, we just have to deal with it, the manuals certainly are never going to be updated. And much of the "updating" information back in the day was only done through "service letters" to the dealer base and the information was never melded into the manuals later on. Until we have some sort of repop publishing of compilations of service letters by calendar year, we will always have big holes in the definitive service/technical information that can only be filled by folks that might have copies of this stuff and are willing to share.
The basic "charge volume" for the r-12 system as used on the Scout II (all versions) is 38oz. The tool of choice back then was a heated dial-a-charge (which I still use on occasion for dispensing r-134a). And of course, those systems incorporated a sight glass in the high side. So a combination of accurate charge volume dispensing, then running a performance test based upon the vehicle setup data described in the manuals while monitoring the sight glass and pressure/temperature display on the manifold gauge set, was the normal methodology used to service the systems.
When I do the r-134a retrofits using the poorboy method, I simply reduce the charge volume by 10% as you have alluded to. That is a "norm" we observed and tested back in the days of developing/engineering retrofit scenarios when r-134a was implemented beginning in 1993 into north American production.
The sight glass has no use in r-134a-powered mobile hvac whatsoever, it means nothing and that is why over the years, it was eliminated from all oem hvac systems. On top of that, the sight glass element itself was a major issue regarding r-134a leakage.
So...if you have a precise method (charging scale) of delivering r-134a into your system, use it! Then simply observe system pressures as the system cycles after fully stabilizing (normally takes at least 20 minutes of runtime at say 70f minimum ambient).
Once the system starts cycling as it should, I think you will see that gauge pressures are right in line with normal r-12 pressures on the high side, most likely less than 15psi greater than the same conditions using r-12. Do not use a shop fan to force air through the condenser when doing this as that will give a totally false set of conditions and pressures.
The oem evaporator thermostat system cycles the clutch at a nominal 46f based upon all the testing I've done. Due to the poor quality of the discharge duct system, the discharge air temps are highly diluted by the time you feel the temp on your face (the tell-tale sensing organ!). However, my research shows that the actual setpoint of that electromechanical stat is supposed to be 36f at "full cold" in order to prevent evaporator icing, and that the exact positioning of the cap tube tip inside the core is critical. So the functionality of the thermostat (whether fixed or variable) is all over the place. This has a huge impact on discharge temps and is the major reason why the performance of these systems whether run on r-12 or r-134a is crappy!
