The Aimpoint Micro with the factory base will allow an almost perfect lower 3rd co-witness, however the front sight hood will kind of be in the way.
You can either get a higher sight or mount, or you can cut the top/middle of the front sight hood off (so its more like an HK416), which is what I did.
Here is a post I made on HKpro about how I measured mine. IMHO this is a good mod to do to the front sight, it not only opens up your sight picture. but I never found it to slow me down when using the irons at all:
As far as I know there were no "official" measurements, if you want I can provide the measurements I used for my front sight:
Measurements were taken from my front sight hood, you can make the measurements a little more closed and then file it open till its how you like. How far open the cut is, is a lot to do with personal preference, so don't think its a set measurement!
The red line at the bottom is the front sight base that the front sight fits into, you can take a compass (or a micrometer if its got sharp points at the front) and set it to the length you want (from the side measurement shown) then score lines on both sides where you want to cut, then just take a fine saw blade and carefully cut through! Finish off with some sandpaper or files, then rub some gun oil into it.
This is a verrrry old pic I took before I completed my USC/UMP conversion:
(note the Factory Aimpoint Micro base)
And a new pic just to show it off
(the ADM base raises it so you cant use the iron sights, but it gives a little more of an open sight picture)