I am doing a number of optical drawings and have parameters defining the lens properties such as aperture, sag, diameter, etc. 

For the lens drawings, it would save me a great deal of time if I could automatically calculate the maximum chip size and populate the drawings with this number.

The formula is simple:

Chip Size = (Minimum Lens Dia - Clear Aperture)/2

Both Minimum Diameter and Clear Aperture exist as parameters driven from dimensions in the model, however, the Chip Size does not exist in the model, it is only used on the drawing.  Can I put an equation in a parameter field?  I've tried just typing it in, but that hasn't worked.

Cheers,

Chris Hatch

PS:  To be slightly more accurate, I'd also like to include the negative tolerance of one of the dimensions in the equation.  Is that possible?