You just have to eyeball the width of the building. I start by drawing a substantial feature of the building (upside-down L-shape on the 13 step), then begin adding the correct number of windows/balconies and adjust the width of the building accordingly. I'm kinda flying by the seat of my pants when I draw my diagrams, I never really know what it'll look like until i'm mostly finished. And yeah, there's a lot of zooming if you choose to draw 1:1.
Maybe try drawing a smaller, more basic building from Windsor first and work your way up. If you post your works-in-progress I'd be happy to try and give helpful criticism. I feel bad for neglecting Windsor.
