As Icarus has pointed out, take the Boeing/Airbus envelope and make it more restrictive at the front and aft and make it your operators envelope based on worse case senarios.
We trim ours by using the baggage in hold 3. Ryanair by blocking the front row of seats by putting the tables down. As for the peeps in emerg exits, the CC move them if they are not able bodied. We do have assigned seating at check-in, but you'll be amazed what scams are pulled at check-in.