I don't know about the fuel consumption figure for a 'bike, but the distances marked on the Michelin 953 are pretty much bang-on.
You have 400Km, Tam-In Guezzam
then 220 Km from there, through Assamakka to Arlit.
On the Djanet-Tam piste, there is 'possible' fuel at Zaoutellaz and Hirafok (so you are looking again at about 400Km between those two points). Whilst a risk for a thirsty 4wd, I would suspect you could get enough petrol at any time from these two places. Large quantities of diesel can be a problem.