Just to be clear, you derived the 42 from the constant I gave you right?
Nope, lifted it straight out of the TCDS
you said we could just use a tape to measure it.
That works fine for other than fuel tanks as you can see the loading position and measure its shape .. from which you can calculate a centroid (or take a stab at estimating it if you are in a hurry).
For nice boxy fuel tanks, likewise, not a problem.
For fancier fuel tanks, either
(a) get the drawing or do the tank measurements and then do the sums - generally not practical
(b) get a fuel tank and, with increasing water quantities (not as smelly and exciting as fuel) run some weighings and CG calculations to develop a table of CG against fuel volume
(c) do (b) with the aircraft - generally the accuracy is not adequate
(d) look it up in the POH fuel loading table and, if the CGs are not cited explicitly, reverse engineer the tabulated data to figure them out. This is pretty standard stuff when developing trimsheets.