how many pounds of freon should be in my heaing system?

I have a heatpump for upstairs and one for downstairs. Heat coming out of the register downstairs is 89 deg F. Heat from upstairs unit is only 72 Deg F and runs constantly on cold nights. My downstairs unit is probably a 2.5 ton and upstars unit is probably a 1.5 / 2 ton unit. Any answers would be helpfull, Thank you.