One of our engineers, Bill Hanna, came up with a calculator using publicly 
available information and from some work by Celestica that was derived from 
IPC. Bill gave the ok to share it, although he notes that it has not been 
experimentally verified. We have the same issue with design engineers wanting 
a lot of grounding, especially for high frequency signals, then we can't get the 
parts soldered and rework is difficult. I used the groupsite file cabinet to post 
the spreadsheet. 
http://ipc-technet.groupsite.com/uploads/files/x/000/046/5b7/Thermal%
20reliefs%20for%20through-holes 

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