On our Perkins M50 we would fill the oil filter as full as we could and take several minutes doing it to allow it to fully soak in. EVEN after doing that and running the engine a few minutes (and topping off the oil) AFTER an hour or 2 of engine use we would need to add another pint of oil. After my very first oil change I was concerned that the engine was using oil (it never did) but then saw no oil consumption. This phenomenon occured after every oil change.
Goes without saying but if you pump the old oil directly into one of the oil jugs with a "site tube" feature on the side, you can measure the exact amount of oil removed and have a very good idea of how much replacement oil is needed.
BTW, thanks for the tip on water ingress through the vent.
Bob and Suzanne, Previous Owner, Brittany de la Mer, SN 86
SM 429 Presently Saint-Pierre, Martinique