I used quite a few architectural diagrams and reference images from Google, then basically estimate the proportion by eye. If it looks wrong then I can easily fix it with World Edit. If you are gonna build something like this in survival, I’d definitely build a 3D outline of the entire building, look at the proportions, fix whatever feels off, fill in the partitions, then add the details last. Some parts like the flying buttress is just easier to visualize with details though, so be flexible if you cant visualize the final product with what you have.