sollution in SAC
input output list: create a parameter (Side)H1 (Type)Double (Direction)Input (Value)-2,00 (Name)height requered. Create Point Py.
Make a Link Py_Px, Type: Delta x Delta Y and assign in Delta Y following formula (H1-Px.Elevation), with x the number of the point.
http://www.c3dbenelux.eu/media/kunena/attachments/26/wandelement.jpg