Round headed openings can be considered as a special case of a circle.
The horizontal you must find is the shoulder of the window. This is the place from which the curve springs on each side. The opening is generally vertical up to this point and then begins the curve.
You need to make sure you have marked where this shoulder is on the template. You should indicate any reference points from the frame onto the template.
The join to the lower part of the window must be made obvious. Normally there will be an overlap between the lower rectangular template and this approximate half circle. You need to mark where this overlap occurs, if you do not fasten the two sheets together. This can be done by marking across the two sheets in a few places. This will enable you to join them exactly back at the studio.