Vacuum and fill by weight is the best / correct method. If you’re still operating weak, or cycling more than it should, running with high and low gauges while filling works reasonably well, but you’re likely to be off pretty bad. Filling the low side while it’s running with a single gauge is better than blindly filling, when I’ve done it I ballpark on the lower end of guesstimated operating pressure for temp, because you’re more at risk for overfilling. If the system is running and you’re under 25psi I think it’s reasonable to add more, but if it’s hot hot you could want up to 55, but filling at the top end of pressures, adding more gives diminishing returns on pressure, that’s when compressors turn into grenades.What is the gauge good for?
It doesn't give pressures on both sides (which is a major way to diagnose problems) and certainly doesn't reveal if the system is properly filled or not.
After watching my father try to destroy two cars with those little cans, I went out and bought a manifold and vacuum pump and do it right.
….so yeah, you can use it to fill between about 25-grenade psi