That is the right idea and how I would do it but the +85 should put the bullet to start a bit either to right or left (I forget which way that is). If the plan is to use this for a 3rd person shooter you are doing good. However in that case I would use vec_for_vertex and put the bullet in the exact spot where the barrel of the gun is.

However you have a +100 and a -100 in your formula and I'm not sure what that would do so maybe I'm way off here.