I have been out of the business for 20 yrs and I didn't keep any of my books on A/C.
For a 3 and one half ton central air heat pump (Trane) totally evacuated system about how many pounds of Freon will it take. I am talking pounds by a scale from a 30 lb can of R22. Depending on the outside temp and other things the gauges may not be right. The old timers use to weigh their cans before and after. One reason was to charge so much per pound for a service call but the other reason was to see if too much Freon went in or not enough on cold days. So what I need to know is about how many lbs approx should my unit hold .
I am thinking 6-8 lbs. IS 8 pounds too much? How much is to high of pressure on the low side? I am getting 58 degrees out of my registers. What happens when too much Freon is in the system. Will the condenser freeze up?

You can weigh in the amount by looking at the model tag on the unit. Usually it is listed there BUT to be honest it is not that accurate. You will need a good quality set of gauges and thermometers and then charge to Superheat or Sub cooling depending upon your metering device.
I am sure you are certified because the EPA has a 25,000.00 fine for all the folks who are not certified for handling refrigerant. Good luck.

The total amount will also depend on the length of your line set and what size liquid line drier you use. Just as hvac1000 said, using superheat and subcooling readings is the only correct way to charge your system. Also don't forget to use a bi-flow liquid line filter drier for your heat pump.

I would assume that you don't know how to use subcool and superheat otherwise you wouldn't be asking, I would say depending on your lineset for a 3.5 ton 8 pounds would be a little much... wait until the house gets good and warm, add what the name plate reads and get a 15 to 20 degree split, that should be close. At 75 in the house don't allow your suction press. To get over 70. Like hvac1000 said if you are not EPA cert. I wouldn't touch it, it would cost you a lot less for a service call then a fine!

How much liquid goes into a unit that calls for 6 ,6 pounds after a compressor change