Posted by:
rroberts This is just a warning, not an error. If the line-spacing for your font looks fine in MS Word and InDesign, and tall letters aren't getting clipped, you are fine.
The problem has nothing to do with the font bounding box size. The wording of this error is misleading, and is a bug in the software library used by FontLab to generate OTF fonts. This is supplied by the Adobe Type Dept, and I will fix it on the next release of the Adobe FDK. The issue is that the TypeAscender and TypoDescender do not sum to the em-square.
It is preferred practice of the Adobe Type Dept to set the LineGap to 200, the TypoAscender to what it should be for the font, and the TypoDescender to whatever it needs to be to make the TypoAscender + TypoDescender == em-square size. However, this rule is not followed if it doesn't fit the font, and it need not be a rule that you ever follow. Just remember what the values are for.
TypoAscender is used by layout apps to determine the distance from the top of the text frame to the first text base-line.
TypeDescender is used to determine the distance from the last text base-line to the bottom of the text frame.
The sum of these and LineGap sets the line-spacing, the distance between the base line of one text frame and the next.
This is all the ideal, pre the OpenType spec, and is not followed by anything other than high-end layout apps, but in the long run we all hope that regular programs will get smarter about layout.
You should read the discussion of the several sets of vertical metrics and the compromises involved, written by John Hudson at:
http://typophile.com/node/13081? 
