Jonathan
Certainly, your method will give a decent indication of the "clarity" of a capture; the more pixels the merrier, for sure.
Unfortunately it is not possible to quantify this method properly since there are no meaningful units, unless you use x number of pixels per square cm or something, which is a decent measure of pixel density, but not resolution.
If you multiply horizontal res by vertical res you cannot express this as a ppi or lppi since the units of this measurement are 'per inch'. It's one dimensional not two.
Any measurement of 2 dimensions must be expressing in 2d units ie per *square* inch- simple geometry.
Jonathan, you are not wrong per se, just talking about soemthing rather different.
The only truly correct way to measure the res of a chip is to do what Phil Askey does and express it as resolved line pairs per inch, for each direction. 2 different numbers to be considered separately.
Nick