I got my scale several years ago including a couple sets of calipers for measuring the line diameter/taper, etc but don't get too wrapped up in the weight as it's a combination of the weight of the first X number of feet,
plus the length of your average cast,
plus the multiple diameters of the various points of the tapers and running line
and more that determines the match between rod and line. Heck, even the air resistance or not,
and the weight or not of a given fly
tweaks the equation a bit!
Of course you already most likely knew all that but whether you use the scale to weigh lines or use one of the old Cortland rod weight determining device*, it is all about that tweaking in the end analysis.
*That was a set of pegs where you placed the assembled rod with a calibrated weight hanging from it's tip, and then a chart behind the area where the tips reached. The amount of deflection on the tip chart tells you the rod weight.
then there is the range over which a line remains a five weight for instance! But then do you measure the first 30 ft or the first thirty feet from the start of taper, or the start of taper plus 6 inches, or the start of taper plus 2 ft?????? All legit questions for certain!