[Phantom]

If you know the coolant temp that turns the fans on you should aim to have enough flow to stay below that at all times when on the highway. For example on a 95F day cruising at 65-75mph going up and down hills my ECT will move between 197 and 204F the thermostat is a 195F. If I stay in that range my fans will never turn on when moving since the fan turn on temp is set to 212F.

One thing you will need to watch out for is Air intake temp as some cars have a fan turn on temp for that also on my car the turn on temp for that is 122F. So if it gets to hot at the intake or the sensor gets heat soaked showing high temps it can cause the fan to turn on.