A normal fill with the system evacuated is around 36 ounces (plus or minus depending on the year of the XJ). If you have a small scale handy use it.
Getting the last of the freon out of the can can be done by sitting the can in a pot of hot water. Keep the water away from the fittings and hoses.
As the low side fills, the pressure goes up, so the cans drain slower and tend to leave more freon in the can. Hot water is the safest answer.
You can also fire up the compressor and as the low side pressure drops, add as much freon as possible, not to exceed the 36 ounce target. A lot of guys screw this up, when the compressor stops, as the pressure in the low side (of the HVAC system) goes up and the freon will actually flow back into the can unless you are fast with a valve wrench.
When finished (this is variable due to many factors), on a 70-80 degree day, the doors open and the A/C on high, the low side should read around 25 PSI.
If the system is overfilled, the high pressure goes up. If it gets above about 250 on the high side, the internal (safety) bypass in the compressor (some models) will open and close causing a heck of a racket. This usually happens above 30-35 PSI on the low side. Some systems (XJ models) have a high pressure cutoff switch, that opens the clutch when the high pressure gets too high and no internal bypass.
You'll notice there are many, about, around or close too or often, in this description and few definitive numbers other than the weight of the fill. And even this changes some from year to year and system to system.
Heat (at the evaporator, at the condenser and outside air temperature), pressures (low side and high side) and the weight of the fill are all variables. With the weight of the fill being the only variable that can be closely controlled.
In most systems there is a site glass, this can also be used. But that's a whole nother can of worms.
High side pressures often doesn't tell you much and high side gages and hoses are hard to use, best to be avoided. Getting the freon stuck in the high side hose, back into the system, is a bit tricky.
Knowing this stuff and teaching it are two completely different things, I hope my explanation has been helpful.
Venting R-12 into the atmosphere is forbidden, they will castrate you and all your male children if you are caught. Having said that, tons of the stuff gets into the atmosphere from leaks or osmosis daily. I doubt an ounce or two is going to affect the ozone hole very much.
