If you have a set of gauges and you want to add R-12, leave the high side gauge turned off and just hook up the low side. I'm assuming you know how to connect the R-12 jug/can to the gauge set. Before turning on the low side valve to add the freon, look at the sight glass on the receiver/drier, you should see cloudy bubbles. If so, start adding freon to the system while monitoring the sight glass. When it turns clear, turn of the freon and you're done. Also, you may need to increase the idle while you're adding freon...I do it myself at the throttle housing location but you may want to have someone sit in the car to do it.
It sounds like you're about a pound low on R-12. Just take it slow and it should work fine.

The low side should be around thirty pounds at fast idle, but when you turn on the valve to add freon, the pressure will jump up due to the pressure in the freon jug/can. When you turn the valve off, the low side will settle back down to operating pressure.

Hope this helps,

Dave