It's the thing that puts me off spending time on this project time and time again. What'll happen is, I get it almost accurate, then try to draw along some axis, and it's just completely off. When i finally fix it, the rest of the scene looks weird, and by the time I come to adding another angle, I'm too sick and tired of messing about with sketchup to bother continuing. The fact is, there's no 100% accurate way to get it right. There is a trick, however. just makes the lines as long as possible, and fit them over the entire axis reference line on the picture.
Oh, also, walkmeshes do help. A LOT. Because you know that they're exactly the right scale for the BG images. I've got junon perfectly matched up now, I think.
I'd say Crisis core maps would be a BIG help if we could get our hands on 'em, too. I have a few, but none of any use really.