When I had a black tank when filling up from fairly empty I'd dump in oil for 5 gallons, fill the sled with fuel, and add the rest of the oil at the end. A few times I didn't leave enough room for the almost quart of oil it'd take so that's why I started putting some in initially. Plus most of it would be pretty mixed up just from filling. I use a boat mixing cup for the ratio. Has a bunch of different ratio marks, holds a quart, and are something like $7.
What do you use the Boat Mixing ratio cup for ?