Remember to use the same units (inches, mm, or whatever) for all of the values!
Diameter is the diameter of your mirror
Sagitta is the depth of the curve you are grinding

When doing Foucault or Rochi testing
you are working at Radius of Curvature or ROC

Check this site out for a drill bit size that you can
use to help measure your sagitta
 http://www.wwforum.com/faqs_articles/DrillChart.html