FTE Pruner has a nice solution which goes to the heart of the matter in statistical terms, and this made me think of the more detailed statistical functions in Excel which can deliver the needed answers.
These are not enabled as a default, and need to be enabled (but worth doing). My own copy of Excel was disabled (thanks to those lovely folks at Microsoft who issue all these updates with disabling powers), so I had to do it from scratch.
Start Excel; top menu: TOOLS ADDINS and then enable the ‘analysis’ ones. On my version, the screen blinked a few times, and then they were working. Then TOOLS DATA ANALYSIS and choose the regression option. Pick the ‘X’ values and the ‘Y’ values, and (before the analysis) I ticked all the options. In the resultant new screen or printout, the “residual options” gave me the predicted values and the error (the so-called ‘residuals’). Very neat.
A word of warning to the serious scholars, Excel is good in this sort of simple analysis (and there are lots more statistical methods in the ADDINS toolpak), and these would satisfy many test pilots/engineer needs. However, they have limitations for serious statistical analysis, and if you are doing a Masters or PhD, or a flight test programme with more than 100 datasets, then my experience says invest in SPSS. Copies can be purchased from eBay at very good prices, and Julie Pallant's book (from Amazon) helps you gets answers straightaway.