The way I was taught back in the 1960s was "Bottom of split ring (Cut line you called it) or just below.
DEKA (Just got brand new DEKA's for house) says either 1/4 or 1/8 inch below the bottom of the filler tube (Same split ring/cut line) Leave a gap.
With time and experience I have came to adopt DEKA's recommendations many years before I bought DEKA's and downloaded their "Owner's Manual"
WHY below the tube: First, the liquid level can go up as the battery is charged or as it warms due to use or sunlight or weather changes.. Second, that slight air gap gives it more breathing space.
If the batteries are low on charge when you top off, consider half an inch below the tube and re-fill when charged.