If you go extra wide for anything close to a 50-50 or 60-40 split you'll just end up needing to reduce the throw enough to get it to avoid being overly sensitive. For a flight trimming function such as found on old timers even 15% of the chord is pleanty. If you want to go with more to make the hinge like end up at the orginal spar location just for appearances then that's fine too. But be prepared to drastically reduce the elevator throw if you end up with a sizable movable to fixed ratio.