So looking at one of the heavier frigates out there who carried a significant number of carronades, took a look at an Endymion class 40-gun fifth rates (as designed).

As designed it was equipped with - 28x24pdrs on her upper deck, 16x32pdr carr on her quarterdeck, 2x9pdrs and 4x32pdr carr on her forecastle.

So that would mean she throws 410# of cannon shot, and 640# of carronades. So would that equate to something like 7S / 7M /4L?

If I'm understanding your formula correctly, I'd end up with something in that neighborhood I believe. I just took 400/100 for long, 600/100 and split it between short and medium (adding to what the cannons gave it)? I couldn't figure out the rationale above for when you bumped only short or short and medium, so I just split the adding of carronade weight between the two range bands - 3S and 3M added to the 4 from the cannons.