How far off are you?

772 and -312
(12 for alignment)

Are you talking about the actual size of your ascenders and descenders or about the values in the OS/2 and hhea tables? If the former, I wouldn't worry about it (though in general you'll want the sum of your ascenders and descenders to be less than your UPM size). As for the latter, you can allow FLS to calculate these values automatically and it will generally assign sensible values.

hey they are actual values of the ascenders and descenders. I've attached the values for OS/2 and hhea as an image. So there is no need to panic right?

Cheers
M

You may find Karsten Luecke's overview of font metrics a very insightful read. His overview of naming conventions is likewise recommended reading. Both can be found here.