The simplest and easiest way to engineer an FSD is to go from a stock "E" grade to an "A". In the process of doing so, though it increases ones range per jump, it also increase the amount of fuel to ascertain that extra range. Actually engineering it to a grade 5 for example does basically the same thing. The range per jump increases, as well as the fuel need to ascertain that jump.
Should one utilize a "White Dwarf" one can increase a single jump an additional approximate 10%; Utilizing a Neutron Star can increase a single jump an approximate 300%. Neither of which utilize any additional fuel.
It would seem that if one gets a flat rate of additional range such as they do by engineering, when utilized, it uses more fuel. If if one gets a percentage rate such as they do when utilizing a star, no extra fuel is utilized.
If the Guarding FSD Booster gives a flat rate up to 10 additional ly's, such as engineering does, then it should also cause the consumption of additional fuel, RIGHT?