Sort of solution for this could be to adjust fuel multiplier by rate of climb..
if plane climbs 3000 feets per minute, fuel multiplier reduces and when going level or diving, fuel multiplier would go to its maximum rate
and that could be refined with some adjustments like if WEP is on, fuel consuming goes with full multiplier amount.
and so on...
well.. just one quickly thought solution to altitude / range problem..
when you climb, you of course don't advance as fast as you do level or diving, so fuel multiplier could be reduced by rate of climb.