That "Dealer Air" system uses an Expansion Valve, and ours was at upper passenger side of the firewall "flange" the doghouse clamps to. All you really need to do about your charge level is see if the low side is high enough to not fall into the Freezing Range on the Temp Scale of your Low Side Gauge, and not too high. Too high will mean that your evap temp is too high. In other words (without a gauge with me to look at) somewhere in the mid-20's maybe low-30's pressure-wise. What's your high side pressure, hot day and fast idle? The more charge you have (above spec) the less cooling and the higher the high side pressure. Hopefully your high side won't be much above say 200. On our converted (R134A) "Dealer" system, we had to run it low on charge or the high side would spike. This is common on conversions, shouldn't be a problem with R12. But that low charge caused our Expansion Valve to freeze over. When they do that, they no longer cool. Wrapping it in that Press-Tite (Cork) tape solved that.
