When cropping how do I change percentage to pixels?

The Crop Tool only shows an aspect ratio.  So although it is possible to create a Custom size using pixels (just type in the number followed by px), the crop will vary in size based on the aspect ratio of the measurement.  If you set your ruler measurement to pixels in the Preferences dialog, when you draw out the crop the pixel size of the crop will be shown:

To get a crop to a specific pixel dimension, you can use the rectangular marquee tool with a Fixed Size measured in Pixels. 

After drawing your desired rectangle, use the Image>Crop menu command to complete the crop.