Ac won't go down past 78 F while operating the presures are 15/ 150 psi. I think it needs charge. But how much charge what pressure does the low side need to be at?

15/150 is definitely incorrect, it's far too low.
There is no 1 temp, pressure to go by, the 40F coil is design condition with a 80F indoor and a 95F outdoor @ 50% humidity.
The more accurate ballpark method is actual ambient plus 30 degrees , convert to pressure for the outdoor unit, so the unit condenses at ambient plus 30 degrees, if a high efficiency unit with a huge condenser coil it may be + 25 rather than 30.Example: 95F ambient + 30F + 125 F , R-22 refrigerant @ 125F= 270#The indoor formula is indoor ambient - 40F convert to pressure.if much lower than 73 inside the coil will be below 32F and condensate will freeze! So you need to get the indoor above 73 minimum or charge to ambient - 40 and hope its ok.
And the sub cooling and super heat is important, 10 degree super heat a foot exiting the ac coil is design with no more than 20F superheat at the compressor as that helps cool it!
So you cannot simply charge every unit to 1 setting regardless of temps and humidity etc.

Old schoolers would grab the suction line(fatter one) and add refrigerant until a slug of refrigerant flowed through their hand, this is not how to do it, but if the coils are clean and all else is fine, it will get you very close. Tract homes are definitely never dialed in or evacuated prior to opening up the system and releasing the refrigerant into the systyem. They have no time to do it by the book. They might sweep it with some R-22 and pull a 20 minute vacuum if lucky! and every home with a 3 ton coil is not identical in size! I see them from 1200 to 1800 sq ft homes, some have 60 feet of lineset , some have 15 ft, they come with enough for 15ft or 30 ft in some. see manufacturers.

The temperature outside and pressure chart are needed to accurately determine the proper charge. (temperature pressure charts to calculate heat gain) However the low side of the unit should read around 70 and the high around 210. This is a really broad generalization and I do not recommend you charge your unit just on this info. It will get you close if you are using R-22 and the unit is properly cleaned. A dirty evaporator coil will cause a severe low pressure reading on the low pressure side.