Luigi,
if you still need an answer, the spring is set by the factory default for a combined weight of cartridge and headshell of 31grams. That is with the classic Ortofon SPU headshell.
When the weight of your headshell and cartridge is 31 g then the numbers on the tonearm weight will correspond to vtf.
You have two options:
a. Just use a scale to get correct vtf
b. Disassemble the spring and readjust to get a balanced arm based on actual weight of your headshell and cartidge. Instructions are available on line.
I just use a scale. Easier and arguably more accurate.
