Can a parametric part used a formtools?

Hello,
I need to used ribs formtools on parametric part. The ribs should always be at the same disance from both edges of the part (centered).
So the form tools need to be link to the part and not the reverse, because the part can be any dimension, so the ribs need to be able to have any dimension too.
Configuration of the formtools won't work as it need to read lenght from the part to get the right size.

Any idea?
Thanks