Start out with 8 quarts and the filter full, if applicable. Put some hours on the airplane, say about 25 max, checking the oil closely to determine at what point the level stays stable. Now the caveat, obviously if the level goes below 4 quarts you better add some oil and figure out a solution to your oil use, like new engine
. But at some point the oil level should stabilize, 6 to 7 quart level with the filter full, and you will find that this is a good level to minimize the mess on the belly and the butt.
Also, is your dip stick level reading correctly? When the oil is drained for oil change and the new filter is installed, as you add new oil check the level indicated on the dip stick. You may find that when you think you have 6 quarts you really have 7. You may need to re-mark the dip stick accordingly or mentally make a correction for that engine. I have seen this exact scenario, the pilot wanted to maintain 7 quart level but really had 8 quarts and grease all over the belly and butt. This is just my opinion of how I operate and many may not agree. Always consult the POH or Continental Engine Manual for the printed information. I don't think Continental indicates the minimum level but Lycoming says to never go below 2.5 quarts on a 8 quart system in the 360 series engine.