Typical 'crack of doom', from British Car Forum:
The original flange - the crack on mine had started where the flange finished and turned round the front of the 1/4-light upright, arrowed (image from Lyndsay Porter's 'MGB Restoration Manual'):
Showing where I welded in two pieces (false colour) to extend the flange alongside and in front of the long leg of the quarter-light:
I happened to see a Heritage shell with doors at a spares place many years ago and thought I could see the flange had been extended round the quarter-light leg as I had done mine. However this clip from a Heritage video (4:33 here) shows it pretty-much as the Porter image and how mine was:
Showing a gap of about 1/8"-3/16" between the side of the 1/4-light frame and the side of the screen seal.
In this picture I have pushed the frame in so there is no gap. Scuttle-shake with no gap like this stands a good chance of splitting the door skin.
With my doors correctly adjusted, and with the 1/4-light parallel to the side of the screen frame, there is a gap of about 1/8"-3/16" all the way up.
As well as the clearance on the side of the frame the front of the frame should just touch the bulbous part of the rubber seal on the screen upright and not press it in as the door closes. (image from Haynes)