mac here , when I really , really want to know , I take a filled Gas can w/me and purposely RUN OUT of GAAAAAAS !
Where it is safe of course , like near a gas station and that exactly filled I gal. in the can. Doin the math(that's the key) you got +1 gal ,watch the light when on , note ist trip odometer reset 2nd trip od etc. may need to take up to 3 cans if you "exactly" trying to figure the fuel light thing . Hope that helps - Ive just figured my exact total tank-ful though , not specifically the "LOW FUEL" thing.