Perhaps for you, Stephen, as you did your due diligence. But for many other companies who may not have the same printer capabilities for repeatability, etc and stencil type and so on and so forth, I think it would be safer to leave the "guideline" as is. 
Good job on that!
Odin

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Vargas, Stephen M
Sent: Friday, March 15, 2019 3:36 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste Size

All:

     A quick update on this issue (I'm thinking this may benefit someone at some time). We were able to successfully print our uBGA boards detailed below. I used a 4 mil thick stencil and was able to specify 7 mil round aperture openings (using type 4 solder paste). Although the area ratio (.438) was below the recommended threshold (.50), our printing was very robust. I attribute this to the Nano coat which my stencil supplier applied to the stencil. Perhaps the .50 area ratio value that I've used as gospel for many years can now be reduced.......

Regards,
Steve Vargas

From: Vargas, Stephen M (US)
Sent: Wednesday, March 6, 2019 4:56 PM
To: [log in to unmask]
Subject: Solder Paste Size

Good afternoon:

    We've got an assembly down here where I need to screen-print down to an aperture size of 6mils round (uBGA). I am using a type 4 solder paste and I'm specifying a 4 mil thick (fine grain stainless steel) stencil with a Nano coat. Boards have ENIG finish. Just wondering what success (or lack of) others are having with this type of application. Might I need to consider a move to type 5 solder paste? Anything else I should be considering? Thanks in advance.

Regards,
Steve Vargas