Using the low side readings will not get you a correct charge regardless of what kind of gauge you have.
High side readings are more acuurate, as are weight /charge ..but the sight-glass on that chassis is where you want to be looking when charging with only a low side gauge b/c it tells what is going on IN THAT system, not what the system Should be doing at pre-determined pressures.
When that glass clears up w/rpm around 1500, you are done.
Any more , and you are over-charging.
I watch the glass when aux cuts in too , just to get it right .

And on these two fan systems, you can up the K9 to 20 amp w/o any consequences. When a low fan blows 15A fuses, always look at the single wire connection on R15 Pre-resistor. [ that is the common weak link on Low Fan circuit] ....and I also spin the fans to make sure they are both free and smooth. Very common for that 15 to blow as the car ages and R15 feed gets fatiqued/corroded. The orig factory wires on the low fan circuit were borderline [ 2.5mm] to begin with.

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A Dalton