you should have some idea of how many miles per gallon you get and also the capacity of your gas tank.
if we assume 35 mpg highway and 13 gal tank
and roughly 60 mph * 6 hr = 360 mile trip (one way)
then we can calculate
360 miles / (35 mpg * 13 gal) = 0.79
so you should be able to just fill up before you leave, and then fill up again for the return trip. but you'll probably stop somewhere (restroom, food), so it might be convenient to fill up during one of those stops. and it's more of a safety precaution if you fill up maybe halfway, because you never know what adverse conditions (bad weather, accidents, etc.) you might encounter that could necessitate burning more gas.
if we assume 35 mpg highway and 13 gal tank
and roughly 60 mph * 6 hr = 360 mile trip (one way)
then we can calculate
360 miles / (35 mpg * 13 gal) = 0.79
so you should be able to just fill up before you leave, and then fill up again for the return trip. but you'll probably stop somewhere (restroom, food), so it might be convenient to fill up during one of those stops. and it's more of a safety precaution if you fill up maybe halfway, because you never know what adverse conditions (bad weather, accidents, etc.) you might encounter that could necessitate burning more gas.