Forcing a Length to be greater than another length

[kmarquis]

I'm setting up a set of door panels families. I'm concerned that the user has the opportunity to type in a value for the various Rail elevation heights lower than the rail height of the one below it. How do I type in the formula to make Rail Elevation Height C a greater value than Rail Elevation Height B?

[Dimitri Harvalias]

You'll need to create a couple of extra parameters to error check this.
See the attached image.
At this time you can't give the user feedback to let them know they'v entered and invalid value but if you can add a piece of 3D text to the family that uses the same formula to control visibility. If the value entered is overridden then the 3D text shows up.
You can also add another parameter so you can specify the difference between the two dimensions