Opening from SFB to Cooking Chamber

I just picked up 3-55 gallon drums. I want to make an offset smoker out of them. I am trying to figure out the opening to the C.C. to the SFB. Using the excel spreadsheet floating around here it comes up with 40 Square inches for my opening. My question is it okay to go bigger on the opening and or or making the fire box a little bigger than the rule of thumb of 1/3 the cooking chamber size?

Thanks in advance.