Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Morgan 4/4 window measurements

Hi there
My cars still currently away having its respray so wondered if I could ask a favour from a 4/4 owner.

Would anyone be able to tell me if when the sliding window is in the open position, how much of the Perspex in front is left not overlapped.

Reason being I want to fit a round 3.5" pop out vent to increase cabin airflow with the roof up.

http://www.ebay.co.uk/itm/281759982494?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Many thanks Tom