Also what helps if you don't feel confident with perspective, is taking the picture you want to draw into 3dsmax or other, and blockout the volumes in 3d; that will help you a lot as a base for your perspective.
If you feel that's like cheating you can use this technique instead, you find where the horizon is in the picture and where are the 2 exit points
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It's the easiest way to do it
Although the real thing would be 3 end points, works well if you want to use focal for something tall, like for example, when you look at the top of a skycrapper from the bottom, the top will be smaller than from where you are.