I fly various survey and ISR aircraft which have non-standard performance due to drag and weight of various sensors and antenna. For planning ferry flights I can’t use the FOM performance figures so I have had to build my own performance models using statistical tools available in spreadsheet programs. INDEX(LINEST) is the most useful - you can develop very accurate models of performance using observed values. So for example, when I start testing a new aircraft variant, I start noting down observed IAS, Density Altitude and power settings over a wide range of density altitudes and speeds. Using LINEST I can then accurately predict what power setting will be required for any IAS and DA. I made another LINEST model for fuel flow against power setting, and so combing them I get range. Modelling climb and descent is more complex but using similar tools and a bit of calculus I got that very accurately modelled too. But now I have a very accurate program that can produce a perfectly accurate fuel log for ferry flights. I can just input the predicted wind, temps, and QNH and FL and can then optimise exactly the best IAS to fly the leg arriving at destination with regulation reserves and my chosen supp fuel.