First post here ,
Would appreciate some advice from the members on my first AR.

I bought a new Colt LE6920 MPS-FDE and a Leupold VX3i 1.75-6 for target/plinking and not sure which scope mount I need for good eye relief and comfortable shooting. I see the 1 piece mounts come in 3 configurations of forward offset(none, little bit, and a lot)...how do I know which model I need ?

I'd prefer a black matte finish, non-QD, mid range price...can anyone suggest a few models for me ?

Thank you

PS: Sorry for the noobie question, been reading and searching for all of my info...just can't seem to find the answer. Here are the scope specs if it helps

Actual Magnification  1.90 x 5.50 x
Linear Field of View (ft/100 yd) 51.40 ft 19.10 ft
Linear Field of View (m/100 m) 17.10 m 6.40 m
Eye Relief (in) 4.40 in 3.30 in
Eye Relief (mm) 112.00 mm 84.00 mm

Weight (oz) 10.90 oz
Weight (g) 309.00 g
Objective Clear Aperture (in) 1.30 in
Objective Clear Aperture (mm) 32.00 mm
Elevation Adjustment Range 42.00 moa
Windage Adjustment Range 42.00 moa

Scope Profile Diagram
A. Length 11.20 in
B. Tube Length 5.40 in
C. Illustration 1.80 in
D. Illustration 2.40 in
E. Eyepiece Length 3.10 in
F. Objective Length 2.60 in
G. Objective Diameter 1.60 in
H. Eyepiece Diameter 1.60 in
K. Tube Diameter 1.0 in
25.40 mm

It's based on what you like.  Just get something that's middle-of-the-road to start with.

Aero

This, they are great low cost mounts.

It took a few tries before I got it the way I wanted it. A lot of personal preferences and trial and error. Go inexpensive until you get it right.

My main point of confusion is the froward offset amount, where the rings are centered/1"forward/2" forward of the mounting base.

How do I know which type works with a 4.3"-3.3" eye relief scope, so I'm not getting blackout/kidney bean view thru the objective ?

For most of us, with a flattop AR and a simple optic it's not going to matter.  There's usually enough play between the mount and the scope and the upper that for most standard commercial scope + mount combinations you will be able to slide the mount forwards or back to find the perfect eye relief.  

I believe that the least extension to fit the better -- even though it's probably not ever going to matter on a 5.56mm AR, I personally like the (probably theoretical vs. practical) firmness of less extension.  

I would get either http://www.graniteridgeoutfitters.com/Aero-Precision-Ultralight-1-Scope-Mount-Anodized-Black-APRA210100.htm
or http://www.graniteridgeoutfitters.com/Aero-Precision-Ultralight-1-Scope-Mount-Extended-Black-APRA210400.htm

The latter is the intermediate size -- some extension but not a ton -- and would give you the most flexibility.  I'd be willing to wager it will work perfectly for you.

And at the sale price, if it doesn't work out you can probably flip it on the EE and find something that fits better and only be out a few bucks (if anything).

If you really want, you can generally approximate things with your gun and your scope in your hand.  Set your gun up with a bipod or rest (or just between a few pillows if you have nothing else), lay behind it, and hold the scope over the gun.  Move it backwards / forwards and watch the sight picture close off and open up to full size and brightness.  Note approximately where on the gun the scope is centered -- if you still have plenty of rail where it's approximately at, then your mount won't matter at all.  If you're running out of room on the upper rail, then you'll need some extension on your mount, and you should be able to tell how much.

The extended Aero looks about right to me.  The 4-14x scope on my AR-10 is mounted forward with a LaRue cantilever mount so that I can still shoot NTCH.

Go cheap first. If you like it, you're good. If not,  you aren't out much $$.

OP, if you are still following this thread, get the Leupold Mark 2 IMS fixed cantilever mount.  Its offset takes into consideration the typical eye relief of most Leupold scopes in the 3.5" to 4" range.  It is sturdy and relatively inexpensive.

When you set up your scope for eye relief, be sure to put it on its highest magnification because that is where eye relief will be the most critical.  You should have enough fore and aft movement within the rings to get good eye relief.  You will usually want the mount to be as far forward as you can get it and still keep it entirely on the upper receiver.  Then move the scope back and forth in the rings to get your eye relief right for your stock.  This will vary depending on the type of stock and its cheek weld.  You will usually end up with the scope eyepiece no farther back than even with the back of the charge handle, and often and inch or maybe more ahead of it, depending on how you position your head on your stock.

a riser mount and rings, they're fully adjustable for eye relief and they're not expensive

Leupold Mark 2 IMS.  It's designed to mount the typical Leupold eye relief scopes on flat top ARs.