Use formulas in family type parameters to calculate values and control family geometry. 
-  In the Family Editor, lay out reference planes. 
 
- Add dimensions, as required. 
 
- Label the dimensions. 
 
- Add the geometry, and lock the geometry to the reference planes. 
 
- On the Properties panel, click 
  
 (Family Types). 
 
- In the Family Types dialog, in the Formula column next to the appropriate parameter, type the formula for the parameter. For more information, see 
  Valid Formula Syntax and Abbreviations .