Honestly I don't use a formula but we fill halfway with backer then remainder with line of choice to right near top of spool. Before cutting off line we then walk off a hundred foot measure we have marked out on lawn. If the line counter reads over we add some line (it calculates based on rotations), if it reads 90 then we are overfull and remove some back onto the supply reel and try again.
It is tedious but it does work. Easier with 2 people.
Are you camping by Erie now or travelling each trip?