You can, just put a little in at a time and see how things go. If you overcharge, it'll most likely just trip the high pressure side switch and shut off the system. Once the system seems to run stable, it's probably pretty close. Don't forget to have the hvac control set at MAX AIR with the blower set on HIGH when testing the pressure.

so i should but more in there. into the low number is in the green?

should i put more in there intro the low is in the green. i am just scared it would blow up lol

Well you need to give us the low and high pressure side at 2000 rpms as well as outside temp and humidity to give you an accurate idea. But it may need more if it drops down to 10 on the low side, assuming there isn't a restriction in the expansion valve or elsewhere on the suction side.