That is going to be tough. We don't know the pan, line size,
oil cooler size,
oil filter, how the dip stick was marked etc.
This next time around I am going to figure mine out exactly but in the past I have used a similar method as petek. I to do not bother with draining the cooler.
I drain the
oil, fill the new filter and spin it on, put in two gallons and check the level. If good, fire it up, shut it down, wait a couple of minutes, top it off. I think if I went and drained the whole bloody thing it takes like 10.5 quarts. But as it is seems to me I add about 9 - 9.5 to get it full with out draining the cooler.
As you can see in just two responses there is a pretty wide range of capacity! If it is full, drain it, measure how much you drained and put that much back in as a start.
Rick