I closed my door, taped a ruler across the gap to keep it exactly where I wanted it and then from the inside I measured the differences between the catch and the edge of the door, the difference between the two inner edges, and just worked it out from there.  Took a while but it seems to have worked.  Saying that I don't have the outer skin on yet so could be way out when I get to that stage.
#7110
Ben Stevens
Keymaster

I closed my door, taped a ruler across the gap to keep it exactly where I wanted it and then from the inside I measured the differences between the catch and the edge of the door, the difference between the two inner edges, and just worked it out from there.  Took a while but it seems to have worked.  Saying that I don’t have the outer skin on yet so could be way out when I get to that stage.