Not sure why you needed 4 steps to taper the faces? I can do it in two steps, and keep the thickness at .25".

When you choose the Taper Face option, you pick two points which define the "hinge" location of the taper, if that makes sense. If the first point is on the back edge, and the second point is along a line perpendicular to the back edge, than the back edge position will not change, and your 1/4" thickness will remain unchanged.

The biggest issue I see with this method is that the edge thickness varies, because you have two flat tapered faces trimmed along a curve, which leaves the top and bottom of the edge thicker than the center.