Certainly in the 737 Jurassics and Classics you need to also take into account the fuel quantity gauge accuracy tolerance which is I believe 2.5% or 3% of the full wing tank reading. With typically a full tank reading of say 4500 kgs that means 135 kgs may not be there. So if you are down to your last 1200 kgs total fuel (30 minutes), at worst case you may have 270 kgs less than you think.