The formula is sort of a pain so just go here Mr Average? plug in the before and then after measurement. In this case you want those measurements to be a month a part. Then Divide the After by the Before and You Got Yaself a Growth Rate Mate.
That's a fun site. I like the graph because you can see a visual of your starting stats relative to average high and average low, and then compare your goal in the same way.
I started big: 220 volume, but with p.e., I've doubled it over six years time. I believe it was Big Al who suggested creating a "paper" cylinder to replicate your penis volume-wise. I'll bet it would amaze some members suffering from penile dysmorphia to see how large their dicks really are.