How low do you let your tank get before stopping?

with my current truck and its 36 gallon tank I find that I tend to go look for fuel when I get right below half - its usually time for a washroom break or lunch/supper anyways.

I'll go further down below a quarter tank on familiar runs where I know I can get there on fuel without issue, like i'm doing tomorrow going from Moncton N.B to Portland ME, I know i'll stop for fuel in Bangor ME as I have enough range to get there and it will be time for supper.