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Take your fuel usage from your post race results per lap. You will see your fuel went down by X.x per lap on a neutral setting as you said. Take the past tracks lap length in Km then divide by the amount of L of fuel you used per previous lap. Your result is your current engine and fuel combinations L usage per Km. Use the L/Km multiplied by any track lap length to get a solid number for L / lap (fuel usage per lap) on a neutral setting. Remember this is usage on a neutral push setting, fuel usage will increase and decrease according to push settings. By using this base strategy, there will be no blame left with a hapless tech director, it will be with you and your calculator.
slight mistake here - use L per lap divided by lap length to get L per Km. Example: I use 3.6L of fuel per lap at Turkey on a neutral push. 3.6L / 5.338 Km= .67 L per Km. Double checking the work we see .67 * 5.338= 3.57646 Round the result up or down to get your final result. You would use .67 or whatever your fuel and engine combos L per Km on any tracks lap length to get a very solid estimate on fuel per lap usage. Remember it is better to error on the side of caution than to run out of fuel.