It will depend on the oil pan you are using. If you have it off the motor then place a flat ruler across the bottom of the block on the oil plan gasket seat. Measure the distance to the bottom of one of the crank counterweights when it is at the bottom of it's stroke to the plane established by the flat ruler. Now you have the distance the crank intrudes into the oil pan.
With the oil pan laying on a flat surface, the opening facing upward and the gasket seat level, measure the same distance you just recorded from the crank counterweight intrusion downward into the oil pan. Mark the inside of the pan at this point with a marker. Now simply fill the oil pan with water one quart at a time until the water level is even with your mark.
This is the max full level of the pan when its on the motor, that the oil is not being dipped by the crank counterweights when running.
Once you have the motor back together fill it up with the same amount of oil as you did with water. Remember that the filter holds a quart but it remains in the filter if it is remote mounted and the openings are facing upward (like mine) If it is in the stock position on the side of a 302 then it will drain back to the pan. Your measurement needs to take this into account and either way you will want to add one additional quart to your fill line to accommodate the oil in the filter when running.
Before you add the extra quart, stick in your dip stick. Pull it out and mark the point the oil covers. I like to use a fine file and scribe the dip stick. This is your one quart low mark.
Now add the quart for the filter and mark the dip stick again. Now you have a second full mark along with a 1 quart low mark.
I don't know an easier way other than just fill it to the advertised oil capacity level and go with that.
engine running the oil level must be at least 2" above the oil pickup, preferably more, along with that it must be below or away from the rotating parts, to some degree those dimensions can be changed depending on baffles, scrapes, windage trays etc, but for normal purposes the 2" dimension should be considered the minimum to prevent air being drawn into the pickup, the reason being in oil dynamics once air is drawn in it tends to creater a vortex ( bit like a reverse tornado or whirlpool effect) and once underway that effect is hard to stop until a wave of oil kills it.