How do I find percentages in Photoshop?

Odd question, I know. But for my school research, I want to take a photo of liquid moving inside a tank, establish a determined area in photoshop, and then have photoshop tell me "how much" of the area consists of "darkened" pixels (the liquid), or if I were to draw a line across the area, how much of a percentage is on each side.

An image of what we're dealing with can be found here:
<http://img49.imageshack.us/img49/2826/edgetq7.jpg>

~HotShotX

Select the area, and look at the Histogram palette. It will display the number of pixels in the selection. If you make a note of the pixel area of various selections, you can then compare them.