I have a workshop that is about 600sq.ft and it is heated with comfortmaker RPJ II. The input BTU's/hr is 45,000. The output is 36,225. I dont have the manual and am not sure if I can add more supply air grills and I suppose if I can I would have to ensure that there is adequate return air CFM's which would mean increasing return air duct plenum ??. What is the calculation or formula for determining the CFM's that this unit can deliver??

I think that each 4x10 grill with a 5" supply line is supposed to have around 200 cfm? Right now I have 6 and would like to add 4 more in the ceiling area.
Will this work?? I like to try and make sure its done right but need to do it myself to keep cost down