The school of thought I subscribed to when buying my mount from Larue was this:
If you have flip-up sights (front and back sight), you get absolute co-witness. If you drop a grand on an aimpoint/eotech/etc. with mount, you don't want your BUIS in the way unless you need it. And if you do need em, you flip em up and they are in the middle of the tube. GTG
If you have fixed sights, get 1/3. That way they are out of the way of the red dot and any possible ballistic drop compensators. And then if you need em, you just drop your head a little. This is what I ended up with b/c my DD carbine has a fixed front sight. And I like it.
This is just the reasoning I found when researching this issue, and I agree with it.
If you have flip-up sights (front and back sight), you get absolute co-witness. If you drop a grand on an aimpoint/eotech/etc. with mount, you don't want your BUIS in the way unless you need it. And if you do need em, you flip em up and they are in the middle of the tube. GTG
If you have fixed sights, get 1/3. That way they are out of the way of the red dot and any possible ballistic drop compensators. And then if you need em, you just drop your head a little. This is what I ended up with b/c my DD carbine has a fixed front sight. And I like it.
This is just the reasoning I found when researching this issue, and I agree with it.