Just the way I do it (works for me). Engine at operating temp. doors open, idle set up a little (a piece of cardboard under the throttle stop) and A/C on high. Pressures are gonna change some (from test to test) depending on the ambient (outside air temp.), high pressure more than the low pressure.
The guages are usually indexed for pressure and temperature. High pressure is largely irrlevant, unless it gets excessively high. Low pressure/temperature should be near or slightly above 32 (35 degrees) degrees. There is often a 12 degree (or more) difference between the theoretical low and the actual air coming out of the evaporator.
Filling by freon charge *wieght* is most always the best way.
If the low pressure gets too low the evaporator is going to want to freeze and the low pressure switch and/or the anit iceing thermostat is going to shut down the compressor. Which will often cause very short cycles (compressor on, then off, then on) sometimes 8 times a minute.
Every time you hook up a high pressure hose, there is a significant amount of freon in the hose. You have to remember to bleed the hose pressure back into the low side. Be sure and purge the hoses a little before hooking them up to get the air out. Air in the system will really run your high pressure up very high, very quick.
If I have to guess, I usually guess on the side of caution and leave the high and low pressure sides a little on the lower side of the envelope.
Excessesively high pressures can cause all sorts of grief and you just about have to plan around the hottest day of the year when it's a 120 out (which it might not be when you do your fill).