I hope I didn't come off as too basic, but I like to try to cover all the bases with A/C since there are literally SO many variables that make a system operate well. I've been active for years on the RWD forums... I'm not totally a newbie to the board, tooting my horn "look at how smart I am, world" :)

First, you can get the A/C temp gauges at a lot of places. Auto parts stores have them, but the radio shack thermometer should be just as good. Anything that you can stick an inch or two into the vents will do the trick well.

You could experiment with adding a bit of refrigerant on the "typical" warm day, and leave a little room to spare. On a sunny, 80 or 85 degree day, you'll have more luck getting an accurate charge and proper operation than you will trying to assume if it's right or not on a cooler day.

Note that you might not achieve "perfect" operation because the system is leaking out slowly, and you are (or will be) recharging periodically. There might be times when the system is a little low, or times when it's a little high. If you overdo it, the system will shut down for several minutes before reactivating itself. That's to allow the high side pressure to relax and the compressor a cool down. If you charge carefully you'll probably find a happy medium where you get very good cooling. Watching your pressure gauge gives you an idea of how well the system is operating. Ideally, having the low pressure side riding at or in the low(er) 20s gives the best cooling, but having it cycle too much on a hot day indicates low system pressure.

With the approximate vent temps I mentioned (yours might vary somewhat), the low side pressure gauge, and the vent temperature gauge, you can get an accurate picture of what's going on in the system. Once you experiment with it for a while, you'll start to get a good feel for what the system's needs are and how/under what conditions it cools best. It's definitely not the best plan of attack (the best would be to put in a new evap) but it should keep you in cooler air for a while.
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Chris Herbst
1992 945, 71k
Volvo shop tech