Well the gable end of this Church is wrong ! I couldn't work it out. So I've been studying it for a few days. In the end, I think I've got it ? The top peak of the gable should line up with the 'middle' of the wall. (I know, that's subject to perspective as well, and it's where I went wrong ). Then where the far wall meets the roof, should be lower than where the near wall meeting occurs. Interestingly, the far edge becomes longer than the near edge ? A bit tricky, but I got that bit right ?
I know, perspective ! You can't really construct it ? In the end you just have to train your eye to know what 'looks correct' ?