Searched but still have questions.

Just how low does the EOTech sit on a flattop upper without riser? (I'm using an "F" marked FSB & Troy BUIS.) I want the reticle to sit just on top of the front site post so it doesn't block too much of the FSB while looking through the BUIS. I also want something SOLID and that will do the best job of not losing zero, even after being removed and reinstalled. Quickness of detachability isn't so much of an issue. How is the out of the box mount option and are there upgrades? Thanks guys!!!

(Currently shopping for best price on a 512.)