As for the Z-filaments, in a reel how many meters of filament are there?
There is some reference to how many meters are we?
You have to remember how many prints have you done to find out how many meters are we?

Around 320m on reel.

Why bother going by length? Get a scale and weigh the role - problem solved.

I make a record of all the prints in a MS Excel table, recording the estimated material consumption of each print, so that I update the (theoretical) mass of the spool after each printing (starting from 800gr). 

It is not very precise. For instance, once I tried to print a piece with a estimated material consumption of 117gr when the theoretical remaining mass in the spool was 138gr, but the print was incomplete  (i estimate the lack of material in not more than 20gr). So now I always take a margin of at least 50gr (I would not try to print a piece with a estimate material consumption of 150gr if the estimated mass in the spool is lower than 200gr).

Yes, I know that it would be much easier weighting the spool by means of a precision scale, but I don't have one.

Thanks a lot!
Maybe it would be better that zortrax would a graduated scale on the reel...