Troubleshooting this, I have found out that "good" values for the  
vertical component of tool-bar-button-margin are multiples of the  
frame char height (div 2). The following figures are examples (good  
values, vs. frame char height):

6 + n*10    20
0 + n*7        14
2 + n*8     16
4 + n*6      12
1 + n*7.5      15

The offset is what I'm stuck with. Where does that come from?

I've tried finding clues in xdisp.c and found it impenetrable.

Thanks very much for any help.