Interesting. I'll be a sample.
I'm 5' 8" 188 lbs. Wrist size just barely shy of 7", and I am not underweight, if anything, I am still over weight. I've lifted before for barely a couple years, but only seriously got into trying to gain size and strength the last 3-4 months.
According to the formula, I will gain 7.35 pounds of muscle by next March.
I am not a hardgainer, I am a hard loser!
I realize this thread is probably meant for general rules of thumb, but I'm interested to see how this plays out. Pull this up in a year and let's see where we are.