My proposal is, just measure dose rate as a function of distance, then use a graph with 1/r2 of the distance axe. You will get linear graphs, which don´t meet the point zero of the x-axis. This deviation is the focus shift.
I want to add, this method is called the inverse square root method. You have to calculate 1/sqrt(D) and plot it against r. If you need some more infos, let me know, I will send you then my results.
Khan´s textbook can easily been downloaded from internet. You will find the third edition containing a chapter about "virtual source" of electron beams.