89 Dakota 3.9. 95 degree ambient temp. Hi and low temps ideally.

At 95 degrees I would expect to see R-12 high side pressure in the mid-to high 200's, low side 20 to 40.

Sorry, its retro fitted with 134a.

The chart I use says at 95 degrees ambient, low side 50-55 psi, high side 275-300 psi

This is it.

Charts based on a dedicated R-134 system might not be relevant for conversions where the recommended charge is only 85-90% of the R-12 charge.

Operating pressures are operating pressures regardless.

its easy to predict the low side pressure, but predicting the high side pressure by ambient temperature is a crapshoot, its very dependent on the generosity of the original condenser sizing, the fan type, and control strategy. We see charging charts based on ambient conditions on commercial equipment, but the chart was plotted by the OEM based on the original gas being tested in every possible temperature/humidity scenario Most of the older cars have a thermostatic expansion valve and with R-134a, I slowly add charge until I stop seeing a change in the superheat and suction pressure, that indicates its getting enough liquid to gain control. The sight glass will remain bubbling, but as the valve gains control and throttles, usually the sight glass will start to fill on its own as the interior cools down and the load decreases.

I don't have the high side fittings on two of my cars converted to 134 fittings because of what you say, high side has a mind of its own. I go by just touch of hand on the suction line at the compressor and monitoring the site glass. On my Rampage if the site glass goes completely clear I have over charged and can feel the temp change on the suction line. Scientific approach to A/C servicing? No, but it works for me.

So, when a dedicated R-134 system leaks off 10-15% of its charge, the pressures listed above are still attainable?

I agree with the sight glass. When mine was bubbling it was actually colder than when it went clear. I thought clear was golden. Should I release (capture) some 134a so it bubbles again?