there's not much to guestimate.
They used 2" tube steel. The top is 54" wide. It looks like the gaps between the steel in the mid-section are probably 2". This leaves you with openings of 12 2/3" for the burners.
Alternatively, they say it's built for 16" kettles, so an opening of 12 inches for the burners lets your kettle overhang onto the steel framing (which I think it is supposed to do) giving you a total of 16" for each bay. This gives you 3 inch gaps between the steel in the mid-section. Since these are whole numbers, I'd bet this is how they laid it out.