I bought a manifold gauge set at Autozone. I discharged the system and evacuated the system down to 27 in hg. I tested for leaks and the system lost 1 in hg in 30 minutes. So, the system is reasonably tight.

I charged with 1 can of Duracool and made the following observations:

AT IDLE (600 RPMS):

The condenser fans turned on at 250 psi and shut down at 180 psi. The engine warmed up, and the electric engine fan kept the high side at 180-190 psi.

The low side was 39 psi.

The fan setting was automatic, and the vents blew 78 degrees. The ambient temperature in the garge was 108 degrees with 43% humidity.

DRIVING:

The average temperature on the road was 103 degrees. At 65 mph, the vent blew 62 degrees on maximum blower speed. At 35 mph, the vent blew 64 degrees on maximum blower speed.

Is this all I can expect?

Do not worry too much about the length of time a vacuum will last in a test... .because many times the gaskets will do just fine in operational mode because they are supposed to be under pressure... they can be pulled away from their sealing surface enough to make it look like there is a leak under a vacuum.... 30 minutes sounds fine...
Other than that I may have to bow out of this because I don't have much experience with charging pressures.... and have NO experience with Duracool... All I have are the charts on R-12 to look at in the manuals....
Maybe you should hail REDFOX and ask his advise on this....

OR maybe change to R-12.... ?

Jimmy, here it is said in a backward manner... but the best test of holding pressure is to test with pressure....