Speaking of 134a ...several cars and truck I've done with this freon have had reasonable sucssess....however, any older car (Especially 911's ) stay with R12 for as long as you can.
From temp' measurements I have made the following is somewhere normal ish' !!
Lo side = 28 to 35 psi.
Hi side = 240 to 315 psi.
Humidity 75 to 90%
This is with the system stabalised, MAX COOLING . BLOWER HI, doors closed, windows rolled up, and ambient temperatures around 80 to 90 degrees F. (I have set my gauges to zero at 1200 feet above sea level.)
The variables are amount of oil in the system. Your vacuum pump quality. Expansion valve 134 compatible and drier the same.
Visual check on evaporator core, where possible, for any obstruction and a blend door system that works as designed.
On systems with a sight glass in the drier DO NOT LOOK FOR A CLEAR SIGHT WITH 134a. it doesn't happen, as the pressure rises the glass will go milky....134a will be fully charged with just a few bubbles runing past the sight glass......I fill the system by monotoring the vent temperature with the vehicle in the shade, ie: no sun load. When the vent temp rises 1 degree from the lowest reading system is as cool as it is going to get......
This is often 44 to 48 degrees F, a few drop to 38 or 36 F but this is very rare ....134a will not chill as cool as R12 system for system...difference is between 6 to 8 degrees warmer..Remember too, you also need different oil for 134a as the R12 oil will not dissolve in the 134a .......
I know there will be some tech's who have their own way and figures, but many of the change over 134a jobs are slightly different and the text book case is quite rare in my experience.