So, what you are after is the roots, then up, then swing to the right and up-ish - the same part I focused on in my suggestions. You could just get a saw now and cut off the part I suggested you layer instead. Go ahead, if that is what you want to do.
But, supposing you try to layer it like I suggested and you get to September of this year and find out you don't have any or enough roots. You have a choice to keep it and try again in 2018, or saw it off. What's the problem?
As far as the part you want to keep and develop is concerned, it doesn't matter whether you layer or saw it off. If you are wanting to get it out of the ground this year, though, all that tall tree will be very difficult to stabilize in a pot/grow-box. I'm assuming you are going to keep it in right where it is in the ground this year.
Regarding the air layer girdle, I suggest that you remove the ring of bark and scrape away all hints of green on the wood that you've exposed. Then walk away --> leave it open to the air for a few days. Assuming you are a working guy wait until the next weekend to 'bag it up'.
The standard thing to do is to pack damp sphagnum around the trunk and cover it with plastic. I use 1 gallon zip-lock bags that I split down two sides. I secure the bottom with a piece of wire, pack in the sphagnum and pull it all closed with a wire around the top. After 2 or 3 weeks, take the wire off the top and genty pull back the sphagnum. If you see little white rootlets, close it back up and look forward to September (you will see lots of white roots in the bag, without opening it, well before you harvest).
If you don't see little rootlets, continue pulling the sphagnum back until you can see the girdle. Make sure there is no growth on the wood. Remove any growth you find. Then close it back up. You're good to go.