position / size arithmetic?

3, but is there a way to do the following? Say I have a picture that is 
50px high, and I want it exactly centered horizontally.  I would like to 
be able to do something like this: 50% - 50px.  This is not the only  
scenario in  which  I wish I could mix and match different types of 
measurements to come to a specific location or dimension.  Especially 
with the standard box model, it would be great to specify a width like: 
100% - 10px, where the border of the box is 5px.  This way one could 
ensure that the box (including borders) would be 100% of the containing 
container. 

Thank you,
Jeremy Nicoll
www.SeeMySites.net