quite easy, and never has refill more than 18.5 gallons. The Light does not have to do with the OBC or the Gauge, I mean, it is not calculated from there, but there is a separate switch in the Fuel sender unit that turn it on. I'm sure there is a post that talk about the Sender unit repair, and shows this switch in some picture.

Don't like the idea of leaving the gas to run out, gas pump is refrigerated by means of the fuel flow.

Javier