As far as I know it is very hard to calculate this. You need to take in loads of variables such as weather (wind mostly) , aircraft , load , exact flightplan. Furthermore you need to include taxi fuel , emergency fuel ect.
That’s a tad long. But what exactly are you trying to do? Know the precise fuel burn at exactly 23487 feet with an 86 percent N1 and a 50 knot headwind and a load of 86 percent? I mean exact fuel burn is only good for that precise moment under the exact environmental circumstances and I’m not sure what formula you’re looking for if you specifically don’t care about flight planning as you stipulated.