hello Alex,

Good questions...

I will go as far as I can.

Starters, I am having a disconnect with figues and chapters you are citing.  
I am working from the IPC-2221, 2222 Feb 98 and 2221 amend 1 of Jul 2000.  
Hope this helps.

Amendment 1 of IPC-2221:

9.1.1 Variable a, add: For external layers, the requirement is the maximum 
diameter of the finished hole.  For internal layers, the drilled hole 
diameter is used.

Annular ring, when required, is as specified on the drawing.  In the case of 
Class three without further callout requirement then it would be my 
interpretation that .050 is external and .025 is internal. [Amend 1 changed 
.05 to .050 and .03 to .025, I assume it was arounding problem.] I say this 
because there can clearly be plane area on external areas as well as internal 
and the 9.3.1 chapter clearly states "These types connections shall be 
relived in a manner similar to that shown in 9-4. …"  It goes to reference 
the "sectional standards" that I interpret as IPC-2222.  Also do note the 
similar...  The IPC-2222 does have a very nice write up on thermals and 
clearances; with the needed clarification of finished or drilled you are well 
on your way to plane thermal and clearance calculation bliss… including web 
and spokes.  Terrific.

As to the .25MM for nonfunc's  I don't understand the question.
B; standard manufacturing allowances and fabriction allowances are the same
I didn't understand the electrical clearance... I begin to think we ar 
reading different specs.

The next to last question you have looks like its out of 2222 and sounds like 
the ole 275 days (mil and IPC).

Your on the right track, I hope you can straighten me out.

Let me know.

Boston Brad
781 858 0783