If R12 in the system, ask older shops that still have a machine that evacuate the R12 or use a vacuum bag setup to remove all of it, even if you have to pay for disposal. R12 is full of ozone depleting badness but it operates very well in it's intended use.
That said, 80% seems to be the rough estimate for the fill of R134a into an R12 system. The R12 system must be empty to add R134a or R12a propane blend type substitutes. I am going through this process now.
The refrigerant charge amount is listed in the FSM - linked right here on the site!
You will also need the chart that shows ambient outside temperature and humidity and at what temperature you should expect the air to be at, when exiting the center left dash vent, given a set length of time and conditions to meet with doors and windows.
I suggest using a shop because they can also tell you after doing the oil return process, assuming your ac currently will operate even if low on charge, and how much oil came out, so you know how much oil to put back in if it tests out that way.
If your system is open to the environment currently, it would be best to replace the drier and you will need a vacuum pump setup to boil out the moisture before installing (oil if needed) refrigerant.