Neil, a small point and others may disagree, but I measure the actual movement of the pressure plate eg. the movement when the clutch lever is pulled right in - first at 2 points diametrically opposite (180 deg) to each other, adjust springs until the movement is equal, then measure at 2 points 90 degrees to the first 2, adjust springs until equal, repeat step 1 and 2, until all 4 points are moving the same, and ALL the springs have the correct (and approximately equal) tension.
After a while it becomes "obvious" which springs to adjust (and how much) that will not mess up the the previous adjustment on the other 2 point, basically I tighten those springs under or either side of the measuring point, to make it move less.
On my s/a 6 spring clutch I get about 080" of movement from memory, I can get the variance down to 010" quite easily, even 005 with effort, the movement will vary bike to bike depending on your cable free play etc.
Measuring the way you did is good, but there maybe an error from inherent "wobble" - due to the clutch not being straight on the shaft or some other reason.
I'm pretty sure the best result, ref reducing drag, is achieved when the pressure plate moves off the other plates "squarely".