Gee, a 50HP motor will likely have two 6 Gal portable tanks, or a permanent tank of 18 Gal or less. It is very easy to get those correctly filled with 50::1 fuel mix.
For portable tanks, run 1 dry, and then switch over. When you are due to gas up, put 1 pint oil in the dry tank and fill it. Now look at the partially full tank. If less than 1/2 full, dump 1/2 pint of oil into it and add 3 gallons of gas. You should easily be able to tell if she is 1/2 full or less. If not, the gas will have an once or two extra oil. Not a big deal.
For permanent tanks, add 6 gal gasoline and then dump in a pint of oil. Repeat until she gets nearly full. Now add 1/2 pint oil and 3 gal gas, until she is full. Again, if you misjudge the gas by a gallon or two in an 18 G tank, it is not a big deal......
Happy fueling...