This is relatively simple to do;
obtain some rigid clear plastic,..enoough to make two pieces,..one to cover v/f front lens and one 25mmx36mm to cover film gate,...this piece make lightly opaque by rubbing flat on mild 'wet and dry' paper then tape in film gate to form viewing screen. Place camera on tripod/support,....tape smaller piece plastic over v/f ,....set on 'B' and look at image on 'screen',...using felt pen, mark position of image on v/f 'lens',......this is the method I used. Also useful to check all your v/f's as you will find considerable inaccuracies in many!
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