You could use model notes to display this info. These can be turned on or off if you don't want to see them. For the note you could type X = &d12 and so on. The information will always be up to date and displayed on screen all the time if you wanted it to be.
Like kschubert recommends, you can create reference dimensions, or use Analysis > Measure and choose Distance from the drop-down menu to measure the distance from one surface to another. If you have a license for BMX, you can create this measurement as a feature in your model, creating a parameter for the value of the measured distance. That way, it's parametric; your model ever changes, the value will update.