Hi Darrell,
Obtaining the value in Photoshop could be considered to be a little "too late".
The idea is that you must achieve this value when capturing the flat field, so you really need to be able to read it from your camera control software after you capture the image.
You would adjust the exposure time to achieve your 30%. For example, if you take a flat and the average pixel value reads 10000, then you would need to roughly double the exposure time and retake the flat.
It all depends on the brightness of the light source being used to take the flat from.
Make sense ?
Eddie Trimarchi
http://astroshed.com