When keystoning distortion is created by pointing your camera up away from horizontal, the image needs to be corrected at the bottom as well as the top to minimize distortion of the image proportions. Only pulling out the top corners makes the building appear too wide.
It is often difficult to visualize what is happening to the picture, so I use temporary guides to help.
Pull a guide down from the top ruler and position approximately half way down. (1)
Pull 2 guides from the left ruler and position one on the left side and one on the right side (2 and 3). Position them so that they cross the horizontal guide right over a good point of reference. (yellow circles)
The goal is to transform the picture for true vertical lines, while maintaining the cross point references in their original location. Be careful to use the free transform tool only horizontally and don't introduce any vertical distortion unless it is truly needed.
Each corner adjustment affects the entire image, so it is necessary to go back and fine tune each one until the image lines up with the guides.
Then clear the guides and crop.
The guides really help me to maintain the building proportions

http://www.pbase.com/image/34561061.jpg

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Joe Filer
Mahomet, IL
My stuff:
http://www.pbase.com/image/33486685