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)