I’ve found this weird bug with the fuel usage slider where it isn’t deterministic what fuel usage you will actually get when moving the slider in SFC. Sometimes you might get high fuel usage, slide the slider one pip left and get half that fuel usage, then slide it right again and get 1.05x more fuel usage instead of 2x like it was before… sometimes if you move the slider all the way left, it will be a little more fuel than one pip to the right… etc…
This is known and, tbh, I’m not sure if it’s gonna be fixed ever. I bet the issue lays in approximate solutions of some fancy celestial mechanics equations. For now, we’ve accepted the ritual of clicking the slider a few times in hopes of getting better flight time 
The issue may be where you guess. They have those approximate calculations in code and can run tests to see why it goes so far off though. The orbital mechanics values between any two ‘refreshes’ of the slider are 1-2 seconds at most, yet you get wildly different results.
It may be that the values they use float instead of doubles or that a double is converted to a float somewhere by mistake. It might be that the slider isn’t actually sending the same value when the same pip is selected.
If it’s a ritual, it’s probably been left for too long. I’m only 6 days in and I’m a little tired of spam clicking and doing the calculation 5-10 times to make sure I get the fuel range I selected.
I can imagine someone with a spreadsheet that already has 5 days of operations planned in advance. Then they accidentally send a ship at the wrong fuel usage and now their ship takes 5 more hours to arrive. Now the ship arrives at 2AM and they have to be asleep for when it arrives… It can definitely cause some madness.
I’ve found that a lot of the time clicking a little past the second pip (about 1/3 of the way between MIN and MAX) will give good results and I don’t have to play with the slider too often.
I think it’s just an artifact of Newton’s approximation method, which is unstable by nature. But yeah, I wish we had more deterministic and convenient slider
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Another advice I can give is don’t load your tanks full for a short flight – the slider will be easier to control if the range is closer to the actual amount of fuel you want to burn