'tankeng' exactly, reed yr Ops manual (if you have one) 'Sunny' correct you need to land with the fixed res intact, all other fuel loaded at the planing stage is usable. Obviously the fixed res IS available should it be needed as a last resort but regardless of what the Regs say IF you find yourself dipping into it then declare a fuel emerg get the flying machine on the ground in a safe manor fill out the paperwork & learn from the exercise.
I've always considered the fixed res fuel unavail at the planning stage, it's there as a legal requirement but also as a last resort to help save yr dumb ass!!.
Wmk2