Not really. You can have a very tiny leak where you may only need an ounce or two that may last for one or two entire seasons. Granted when it's running is when it'd leak the most.
Don't forget about the orifice O ring where the piston is. A lot of times you can simply tighten it down. Others, if the seal is melted as in when brazing in, you're going to need to perform a full recovery or pump down of refrigerant and open up the piston and change the ring.
I'll have to get some 410a and top her off, then find the leak
Hearing that it's typically should be around 120 suction/375 discharge for 410a
Roughly, yes and because it's a fixed orifice that 125 running pressure (the temperature of saturation inside the evaporator coil, the temperature the refrgerant is leaving the evaporator at in gaseous state) would be 42-43-ish degrees f. You then take the low/suction PIPE temperature at the condenser and subtract the 42 or 43 from it. That's your amount of superheat, how much heat the gas has gained. There has to be a substantial/specific amount of superheat calculated simply by the way the guy in the video explains for proper charge or the gas can turn back into a liquid in the suction line.