using a reference plane position in an equation

I am creating a part using loft, "morphing" one sketch into another, each sketch in it's own plane. Is there a way to parametrically define the dimensions of the current sketch based on the x-axis position of the current reference plane? For example, if I place 10 reference planes, each parallel to the right plane, and 50 mm apart, is there a way to know the absolute or relative X (or Y,Z) position of the current plane, to be used in an equation describing the current instantiation of that curve? As a simple example, place a circle in the first plane, and I want each subsequent circle on the

Any Ideas?