my new roof ribs are in

well I got my new roof ribs in, now I am ready to make a pattern of my roof out of construction paper, so I can then transfer it to the donor roof.. but I don't know how far forward to cut the donor roof... all the way to the edge where the windshild is or back and inch or two... I have a new visor to put on as well... the new roof is ribbed so I am going to have to cut and bend the ribs down in the front I know that will shorten it by just a bit... I am just unsure how far forward to carry the new roof... then I plan on setting the roof on and making sure the ribs support it and it is level with the sides of the car if not I will shim the ribs till it is... before doing any "spot welding" I am just puzzled how far forward to cut the donor roof can anybody help with this???

thanks,
simon