If you're going to be doing this a lot and are prepared to drop $100 for the book and parts to build your own, you might also consider buying the special SFIC vise for the machine:
http://www.hawleylocksupply.com/Merchan ... de=F-PartsI bought one for mine, and found it to be worth the cost. The other nice thing about it is it comes with a builtin tip stop, which is how SFIC keys are measured, so it also eliminates the need to "reverse engineer" the spacing from the shoulder.
If you go that route though, I'll give one word of caution. Apparently when Framon builds the machines, they custom fit the vise to the machine, so if you add an after-market vice it is possible that it will register in the carriage such that it's not perfectly parallel to the axis of motion of the machine. At least this is the case for the older manual #2 machines like I have. I spoke directly to Phil about this and he confirmed it. So you may have to slightly file the vise to adjust the registration, or simply correct for the error by adjusting the depth accordingly as you cut the key. The good news is that at least on mine (and having spoken to Phil and knowing the underlying reason, I suspect this would be the case universally), the error is constant and predictable, so it's easy to adjust for on the fly.