I would like to share a tip ad hope this would be helpful for someone in the projects. One of my project required to maintain the inner radius of the duct elbow at 150mm regardless of the size. A small trick in the formula helped me to acheived it.
When I edited the family to modify to meet the project requirement, I found that the radius of the Elbow fitting is derived by the "Center Radius" of the Fitting. The "Center Radius" of the Fitting is then derived by the Radius Multiplier
|Image showing the default parameters of Duct Fitting|
|Modified Parameter to maintain the Inner Radius.|