My understanding is the pressure with the engine off (called static pressure) is NOT an accurate way to measure the amount of charge in the system.
Here are a couple of good articles. I am not a professional, and I would defer here to anyone that is. I have worked on a few of my own cars with a/c so I am mostly talking from experience and/or what I have read!
Here is an article from another forum:
See especially post number 13 - 15.
And some more in depth reading:
I would say if your compressor is kicking out and the pressure drops to around 25 within about 5 seconds, then you are low on Freon. It should drop down to 25 and kick the compressor off, but it should takes much longer than 5 seconds, even on a 75F degree day (Somebody correct me - does 25C = 75f?)
The normal way to check the low pressure is with the car idling (some recommend slightly above idle - around 1200 RPMS or so) a/c on max, fan on high and doors closed. Let the car idle for at least 5 minutes or so.
Many gauges will have a chart that shows what your low side pressure should be as compared to the ambient air temperature. As you may know, the higher the ambient air temperature, the higher the low pressure side will be.
I might also suggest many places do a relatively inexpensive check of your a/c system for like around 30 bucks or so. Might be worth it to have them tell you what the problem is and then go from there.
I also might state - I don't know how much experience you have - but working on a/c systems CAN be extremely dangerous! It's a case where a little knowledge can INDEED be very dangerous!