R134a adding refrigerant

I have a 98 Cherokee and the a/c hasn't worked for well over a year. So I bought a service kit to add refrigerant. I followed the directions and still need some help. I connected the gauge to the port near the firewall and the PSI reads 45, I started the Jeep let the A/C run for awhile then connected the gauge and it will build pressure up to 45PSI and then the compressor kicks on and steadily drops it to 0. From what I can tell this is where the pressure needs to be. I don't want to overfill the system so is this right?

Thanks

Mike

Here is a table,kind of hard to read, all your interested in is the ambient and low pressure parts of it. http://forums.audiworld.com/4000/msgs/8184.phtml
My shoot from the hip guess was between 30 and 40 PSI, the table bears out my guess, so it seems OK. I´ve never worked with 134a much, but have the tables memorized for other refrigerants.
Low side pressure changes, with the ambient (outside temperature). Filling by wieght of refrigerant is most always the best methode.
I usually fill with the doors open and the A/C on high, then fill till the return line to the compressor, near as pratical to the firewall, starts to feel cold (finger test).
If you have a refrigerant temperature table ( a pressure gauge with a temperature scale and PSI scale), the low pressure will always be above 32 F.
But being afraid to over fill is a good thing. Being quick with your valve and closing it off quick, if the compressor stops (it will build enough pressure stopped to sometimes force the refrigerant back into your can, on occasion). bleed a little refirigerant into the lines to purge the air, before connecting them. Air inside the refer system, messes with the pressures.

When I filled my 98, it red zero until I got some in, then it did just what you were describing. I added the rest of the can, and the compressor quit cycling on and off. It read a steady 25. Been working for a few weeks now.

It's never gonna work great because you have air in the system. Those DIY kits make it look really easy to do and all but they leave out the step that if you have leaked out your refrigerant then you have air in the lines and it needs to be purged before you put more in or else it don't work well.