"bowls" as in helms?
Sit the "bowl" (edge down) on a flat surface.
get it so that the crest-line runs nice and vertically and it has a pleasing shape. Prop it in that position with matches or little wooden wdges (halves of wooden pegs).
Take a marker and lay it on the flat surface and circle the "bowl".
If you hit a gap then just raise the marker by laying it on a piece of cardboard or a matchbox or something. Draw your line by sliding the marker all the way around the helm-top. Now you have a nbice even and smooth edge.
Much easier (I think)
Sasha
P.S Reinhard. Thank you for getting those pictures to me.
You have a talent for making mediocre quality stuff I built look
GOOD 
I will resist thge urge to ask you to take pics of some of my old helms down there...I don't think anyone's photography skills can save them.
