How to make a dimension equal to a driven dimension

Hi everybody,

Is it possible to make a dimension equal to a driven dimension?

To make myself clear I am attaching an image. On that sketch, is it possible to give the horizontal distance between tangent point and sketch origin (indicated as a reference dimension of 53.9) as a driving dimension for the length of horizontal line (current auto dimension is 78)?

I tried measure (please see video clip) but it didn't work.

Thanks,

Ural