Digital BW, The Print

I knew I should have paid attention in my high school math class.  
Figuring out matting and framing sizes is driving me nuts.  I have, 
say an 8" x 10" print in landscape and want a 1/2" border around it 
between the image and the mat for a signature at the bottom.  I'd 
also like to bump it up a bit so the bottom mat border is wider than 
the top border.  Maybe an 1/8th inch?  I'd like it all to fit in a 
standard size 16" x 20" frame.  When I do the math the top and bottom 
mat border is not as wide as the side borders or vice versa if it's 
in portrait.  Does it matter?  Figuring out the cutting dimensions is 
a pain no matter what the print size.  Is there a table somewhere 
that will give me the final cutting dimensions based on the input of 
image size, border and mat border?  I bet it could be done in Excel 
if it isn't already.

Thanks,

Tom