I have not seen any pointer on the question I am going to ask. 

When I cut/scrap off the bark to air layer is there any particular place that will root better than others?  Or is it all the same regardless where you put it? 

I keep looking at my tree with guilt like I may kill it and just stop my shelf from doing it....... I had the bag, the string and utility knife ready...then stopped

I've air-layered without cutting any bark off. just make sure you cover 1 or 2 leaf nodes with the soil. I watched this guy from down south on youtube do it as simply as possible, look for it

I just take off 2 to 3 leaves,
wrap that area with damp sphagnum moss,
Cover with plastic wrap, then foil to block the sun.
In about 4 weeks you will see roots, but I leave them on a few more weeks to make sure there is enough roots to support the top growth.
No rooting hormones are used and I've had success. Hardy Chicago seems to be on the stubborn side to root but others did well.
Hope you have good luck.

So nobody scars or cuts the bark like in the videos.  Tonycm, why do you take off the leaves?

Kevin, I scrape the bark as if I were using a potato peeler in three places around the bottom part where I will later
cut from the mother tree. I use Rooter Pots and Q-Plugs for my rooting medium and rooting gel is optional.

When I take the leaf off it leaves an open area similar to cutting the bark but it's more of a natural open area.
That's the place where I wrap the moss around to form roots.
I figure since it's more of a natural scar, if it doesn't root then you can remove the moss and not leave an open wound where infection can form.
Whether or not this has any science behind it, it works for me.
If it's not broke, don't fix it.
I try to keep things as simple as possible.
Give it a try.
If it doesn't work, simply remove the moss and the branch will have no wound to heal and it'll keep growing as before.

Thanks for the good pointers.  I will wait a little for I start to layer.  I will practice with my lime and mangos a bit more.