Your avatar photo shows that you have a perforated aluminum toe rail. My spring line is attached to my toe rail with two bolt type carabiners. No need for a mid-ship cleat. If you still want or need to add mid-ship cleats though, you could experiment with a spring line attached the toe rail to determine the optimum cleat location.
By the way, I use my toe rail spring line only to get on and off the dock; not for long term docking. That’s done with dock spring lines run from a mid-dock cleat forward to the bow cleat and aft to the genoa winch. Don’t want the annoyance of carabiners rattling on the toe rail at night.