I have never done it but believe it can be layed out using parallel line development. If I get a chance I'll make a drawing. The only thing different is the fact that the ordinates for the ell will be transfered to the point of intersection by swinging an arc from the center of the radius of the ell.
Draw an end view of the header showing the header and the ell. Draw a projection of the,end of the ell. Divide that into 8 or 16 parts. Then from the center if the ell, draw an arc from each ordinate to the point of intersection on the header. You then have something to start with.
Transferring the lengths for each ordinate would be a pain I think.
Of course some hands can probably just rough it in little by little.
This is from my phone so may have some spelling and other errors. its a pain to edit after something is typed.
Gerald
EDIT
Here is what I THINK you are doing.
I will try to show what I was suggesting for the layout.