the overflow doesn't determine head loss. height and elbows do. in other words, if i had a 120, i'd probably go with an overflow box between 800-1200 gph
. once i picked that out, i would pick a pump that after about 4 feet of head, would equal my overflow.
you don't have to be exact- you can always choke back a pump safely at the exhaust port with a ball valve or gate valve if need be. you just don't want a pump rated for less than your HOB
overflow, because it's possible that over time it can lose siphon.
head loss is figured out by taking into consideration height and turns/elbows.
there is no head pressure sending water sideways.