I judge my optic height based on my iron height. I like a full co-witness so I'll raise it up to match. If you have fixed irons that you cannot flip down, you may want to consider 1/3 co-witness. Generally, for AR, the height of the irons tends to be the minimum most ergonomic position for whatever you have mounted. Any lower is awkward, any higher depends on the shape size of your head. Even with a large head (like me) any higher then 1/3 co-witness, you'll loose your proper comb/cheek height unless you get a cheek riser for your stock.
An extender lets you mount more forward, not really needed unless you're running a scope and need to set it for an ideal eye relief.