This is just a Cessna 150 driver talking (so far).  I may not fuel up at all, but if I do, I'll fill it all the way.  I never saw a point to filling part way and I don't even pay attention to any extra costs associated with carrying unneeded fuel.  I am not an airline that lives and dies by "pennies per mile".

 

I don't have a hard and fast rule, but a hour reserve sounds good to me.  Less if I'm just staying in the pattern.  But again, tht's just to determine wether I need to add fue at all.  If I do, it's all the way to the top.

 

I recently set my low-level sensor in the Lancair header tank.  I set it for 30 minutes at cruise fuel flow.  (IO-360, 8-10 gph, so 4-5 gallons)  This is for the big red warnig light that says "You're almost outta gas".  I figure that I can cover a lot of ground in 30 minutes.

 

- Rob Wolf