You should have lower than 50 at idle on low-side, the low-side pressure should not change that much from idle to 2000rpm. The belt is still loose. Also are your aux fans working properly ? The pressure is a little low at 2000 rpm, get it to between 35 and 45.

One way you can optimize your a/c is as follows: (it is not perfect but works better than trial and error).

1. You will have to either fix the belt OR keep the car running on 2000 rpm during the test below.
2. Run the a/c for 10 min on re-circulate mode with highest speed for blower, lowest temperature selection and some a/c mode for air routing, e.g. bilevel.
3. Now read the temperature (degF) corresponding to whatever low-side pressure you have from the gauge. There is s seperate dial marking for temperature corresponding to the R134a on the gauge.
4. You will need a infra-red thermometer or a thermometer that you can wrap around the low pressure hose near the low pressure valve. Measure the low pressure hose temp (degF)
5. Adjust the pressure on the low side by charging or discharging until the temp measured on the hose is between 15 to 20 degF lower than what is read from the gauge.

Maybe your belt tensioner is not working and so you cant tighten the belt properly. Also make sure there is no oil or grease on any of the pulleys and the belt.

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Saumil S. Patel