General question - O-ring size selection
 

I'm faced with finding an o-ring "off the shelf" for a particular application where the part is no longer available from the manufacturer. In this application, the o-ring sits in a circular groove that is rectangular in cross section and mates against a flush surface. Is there a rule of thumb to follow in selecting the cross-sectional diameter of the o-ring to provide a sufficient size to fill the groove with an excess that gives adequate compression to assure a good seal?

As always, thanks in advance.