Chrysler and other auto companies push the Evac and recharge to help the dealers pay for the Acid Head Hippie EPA mandated equipment & procedure, on the backs of the customers. This crap was brought on by Junk Science funded by DuPont and sucked up by Green Peace. Of course the Elected Idiots of both parties voted it into law or gave EPA the power to do so. Arresting AC shop owners/techs who leaked refrigerant into the atmosphere was the lead story on Local News and CNN more than once.
If you put too much 134A into the system unlike the old R-12 it will hurt the performance instead of help. What the gauges will tell you if you hook them up is that you are generating pressure on the high side with in acceptable limits and that the low side is significantly lower. My gauge set has markings on the gauges (in color), but since I don't carry them with my luggage, I can't tell you the pressures off the top of my head. Since the laws of physics that apply to 134A are universal hooking the Gauge set to a car with the same type of AC system (orifice tube and expansion valve are the two most popular) that is working great will divulge the Classified Intelligence on the proper gauge pressures, just in case you don't trust the gauge colors.
Autozone, Advance, & O'Reilly sell Cheapie recharge low side color coded on the dial gauges that work for those who have back trouble and/or hemorrhoids and can't bend over at the Stealership or Ch-Ching AC shop. The other great thing about the Cheapie gauge/recharge tool is that it comes with instructions in at least three languages
When I was converting R-12 to 134A systems, and did not have the proper weight for 134A, I would mount the AC gauge set so I could see the gauge dials through the windshield and take the pressure readings going down the road with the AC on. When the down the road high side was at 175PSI, I quit adding any more 134A, worked great. Again this trick was for systems that were converted from R12 to 134A.
The thermometer test works too, in fact I leave a thermometer in my dash vent year round so I can find it or get board waiting at the light and want to know how my AC is working, turns fewer heads than the Gauges on the windshield
For our Friends across the pond, check UK eBay. For some reason lots of stuff that hits you in the face when you walk into a US Auto Parts Store, you can't find in the Auto Parts stores over there, is on UK eBay. My son who lives in the Netherlands for his Job and his Squeeze, gets lots of his stuff he needs from UK eBay.