Take note that the idiot light set up is only for idiots to remind you that you will need fuel sometime soon.
I've reset mine to come on when just under a 1/4 of a tank of fuel if left, this envolved removal of the complete assembly from the tank and alteration to the unit - I can give no advice on doing this as it envolves resealing of the fuel tank - situated above a hot engine.
However most of you would I expect stick to the proven method of using the trip gague, zero in when you fuel up and keep an eye on how many miles you've done, its that easy.
Me I use both - never run out yet, but been very close in Sweden last year with 206 miles between fuel stops.