[cadcam]

So you know renton Z is below the top of the part. his way of making a line tangent to the arcs is good.

Now to make sure you get the highest parts. Try using this option.

Create
Next menu
Boundry Box

this will make a box around your part at the highest parts. then you use the
Xform Translate
between points
pick the upper left hand corner and type 0,0,0 being X0,Y0,Z0 location.