Parametrics of segment cross section

How can I constraint a geometry point to more than one parameter.
The circled red point varies due to

  1. web thickness and
  2. bottom slab thickness.
  3. fixed 250mm from bottom slab

Tried the below, but it is not working.

Cheers,
Derrick

You can get more information about parametric definition of point based on multiple existing other points under AQUA-manual, chapter 3.33.1Parametric Sections