Ray, I believe the formula is correct.
If you  poll some of your pwb vendors I think you will
find that they will request , if not require, additional clearance
on the inner plane layers. If , due to a miss drilling you have
tangency or breakout on an outer layer pad if may not render
your boards to scrap.
But if your plane clearance ( anti pad )is the same as your outer pad
and you have break out, you will have shorts to the inner plane layer.
The additional clearance of the anti pad will provide a better yield for
your pwb vendor.



Tim Easterling
Cadd Manager Commercial Engineering
SCI Systems Inc.
8600 S. Memorial Parkway MS 200
Huntsville, Al. 35802
(256)882-4426 (ph)
(256)882-4700 x4426 (voicemail)
(256)882-4304 (fx)