I'm after curving some decking steps thus...

Your looking at the decking, 4 steps left, 4 on the right. each step/ tread on the left is 250mm deep, each on the right 500mm deep, space in between 4m.

I need a steady curve from 0 to 250mm ( which is the difference between 250 & 500mm ) over a distance of 4 meters. Say at an equal spacing every 400mm.

Is there a formula?