That is one way of doing it, but it usually takes a lot of time: Run milling job, guess new angel, repeat.
Using a tramming bar is no new invention,. It’s been used for years (forever?). Both on manual milling machines and CNC.
Your can also use a piece of glass like this: https://www.youtube.com/watch?v=DmuipSuDNAs
A carpenters level is just not accurate enough for CNC (it is for door frames). If you manage to get the bubble 100% centered, most (if no all) carpenters levels will only give you +/- 0.5mm over 1m. Looking at your pictures you don’t event get the bubble centered.
Like Phil said “It’s not about being level to the earth” (unless your are milling water). Don’t use cheap bubble levels for this. If you have to, you need to buy something like this: https://www.amazon.com/dp/B0002DOKOC
BUT if you are happy, I will say no more 
That is $849.99 which most of us do not have.
I never said it was accurate.
It will get you close. After that you have to use a dial indicator and tram bar as was already suggested.
If I had the extra cash I would get a digital level and set things up with more accuracy. I just don’t see the point on a hobby machine. $849 can go towards some other things.
This one is cheaper
https://www.amazon.com/dp/B00HX2MKOC?psc=1
Definitely - I need to make/buy one of those, just haven’t gotten around to it yet. Like he said in that video, by far for me the most annoying part of this whole thing is the add-the-shim-and-test-again part, since you need to keep unscrewing (some, not completely) the Z carriage to do the shimming. Interesting that the much more expensive machine he has in that video has to be shimmed too.
Since performing this adjustment I have had no issues with carving. I even do not have to run the wasteboard skimming as its very close.
Trivia question: What’s the flattest thing on earth?
American wages.
