18 mmThey're meant to be short but it's up to you, did you specify a certain length or just ask for an extra 3cm? It's usually a good idea to compare measurements to your current well fitting jackets. What thickness sheepskin did you pick?
And yes I specified a 64 cm of desired back length (we didn't talk about front length as I simply assumed that it should be lengthened in proportion of the back). My Levi's leather trucker jacket has a back length of 62 cm and its front length is about 4 or 5 cm longer than this B3 jacket there. Which is quite odd. Did they forget to lengthen the front length?