The exact metric size packing that Amel ships can be the very devil to source in the US, but the closest inch size works fine. If I recall Amel supplies 8mm packing, and 5/16 inch packing is within 1% of that. But measure before you buy, my memory isn't 100% on 8mm or 10mm...
Far better to cut by wrapping it around the shaft than to do it by measure. If you do perfectly square cuts on a length of packing exactly equal to the shaft OD, the cuts will gap apart on the outside of the packing with is significantly longer in circumference than the shaft.