Hello again, a lot of the guys are using the Testo 550 instrument to charge systems with, especially heat pumps. see this video
Super heat and sub cooling in the 6 degree range should do it for the system you have. If the delta T on the indoor coil it still a little low the cause is likely more air flow than the system is rated for in heat mode.... combined with inevitable1 to 3 inaccuracy of the thermometers you are using.
If your sub cooling is much over 6 degrees on the indoor coil, under the conditions you describe, the system is likely over charged by a few ounces....
My experience has been that its best to check the refrigerant charge again in the summer, it seems to be easy to get a slight over charge in heat pumps in the winter... then in the summer that runs the suction pressure
up high enough to ruin the system efficiency and run the electric bill up noticeably. (because humidity/ latent heat loads are an issue in the summer, but not in the winter to the same degree.)
I would not charge the system to the air flow delta T across the coils, but only by super heat/ sub cooling... on both the inside and outside coils under the conditions you describe.
good luck with that sealant. Goodman coils are well known for leaks. They use thinner copper in the coils than some other mfgrs. You may want to look into a warranty replacement.. at 3 years its likely out of warranty, but Goodman has been pretty good about replacing leaking coils... it would be worth a try. if the leak is where the tube expands into the galvanized tube sheet, its cracked...and will get worse over time.