Competent engineers should be expected to be able to work accurately with either.
You're absolutely right, it's not a complicated calculation. But it *is* easy to get wrong with very little indication that you've made a mistake.
Someone who's sat comfortably at a desk, during the day, with no time pressure is able to complete this calculation repeatedly and without difficulty. At 3am, cold, hungry and covered with JET A1 is a totally different proposition.
At this point there's not necessarily a huge point in making someone else repeat the calculation independently, because two people can get it wrong. Putting a big label on the can that says "this treats approximately 50 tonnes of fuel" is a bit of mental maffs that anyone can do.