You need 3 things, nozzle size, nozzle spacing, and true ground speed. The link to the online spreadsheet is set on T-Jet nozzles.. Just pick the nozzle you have, nozzle spacing and the graph will tell you ground speed etc.  It's very easy to use.

http://www.wyliesprayers.com/broadcastTips.php

I highly suggest you use a good spreader/sticker surfactant IN ADDITION to the roundup/dicamba mixture.  You can kill alfalfa very easily with 24D-dicamba but,, you have to get the chemical thru the waxy surface on alfalfa. Do not depend on the chemical's being able to get into the plant all on their own,, a good surfactant is much cheaper and greatly enhances the chemical's ability to kill the plants at a lower rate even.

I know the chem companies always say how they have all the "additives" needed for a good kill.  B.S!  The companies don't put in enough surfactants to allow the chem to get thru the surface on most weeds. (which the alfalfa is at this point, since you are intending to kill it) The chemical companies always test in "optimal test plot" conditions, not real world conditions.