We are going to make some changes to this calculation in the next update, so instead of setting a fixed amount of liters or gallons in
"Additional fuel to add to finish the race"
it will be "Additional fuel to add to finish the race (number of laps worth)", the value can be in both whole or fractions of a lap(s).
"Enable tight fuel calculations" will be removed but setting another value then "0" to
"Additional fuel to add to finish the race (number of laps worth)" will have the same effect as having it enabled.