As a Brit living in the US it took me over a year (and several misunderstanding) to realize that there is a difference in the use of the word 'punt' between US and UK. The main confusion was between:

  1. (Informal) To cease doing something; give up [1]
  2. (Chiefly British Slang) To gamble [1]

Often the context allows for either of those to make sense. Off topic, I know, but maybe it will help others avoid the confusion.

1. http://www.thefreedictionary.com/punt

Yes, you could go on to create a chart that you could extrapolate future readings from, but I was just trying to answer the question of correlation between photo artifacts and actual radiation.

You'd want to go back with a plan and find a wide range of different radiation levels to get a calibrated scale worth using. Your iphone almost certainly has a wacky non-linear response to gamma rays.