I can even have a guess who the Captain is.
Its not common to call it granny fuel.
19 seaters are in the main under 7500kg So we are not talking 200kg extra I would hope. Maybe 50kg which is 10mins extra.
Some round up to the next 50ltrs of fuel. Some use 0.77 SP when ordering and some round down the arrival fuel.
Its all OK until you have an engine failure in LVP's and you can't look out the window and see and avoid. These machines are in general getting on for 20 odd years old and they weren't very spritely climbing on one engine when new.
Also another thing to take a look at is the drift down which is surprisingly low at max weight. So you may have issues with reaching MSA if you have already got 18 blokes on-board who last saw 86kg in there teens.
Personally I just don't, I check the MPI's to make sure they are the same as the gauges and then fill up to the required amount. No more no less.
But there are more than a few out there, that to be honest are correct reckon that your more likely to have issues due to low fuel than an engine failure when flying a route at min fuel and Max load. An extra 50kg isn't so bad its when they start going 100's of kg over weight or in LVP conditions it seems a bit crazy to me.