might be showing your youth there ems R22.
the standard used to be, make the booms as long as the helicopter could handle. poke the spray right out into the vortices to get as much spread as possible.
then people started to get worried about spray drift, lifestylers moved in, sensitive neighbours came, etc. everyone had to go down to 80% of rotor diameter.
now i guess that some people are looking at extending booms back out again.
it depends on lots of factors, but if you want to spread the stuff as far and wide as possible, then poke the spray right out past the rotors, if it needs to be more controlled, then 80% of rotor width.