If you want a overdraw 'debug' shader, you should simple render the scene using additive blending with a static color (for example float4(0.1, 0.1 ,0.1 ,1) ).

places with little overdraw will be quite dark while places with a lot of overdraw will be white.

In case everthing is black or white (the minimum/maximum), simple adjust the fixed color. for example change it to float4(0.25, 0.25 ,0.25 ,25) if it is to dark, float4(0.02, 0.02 ,0.02 ,1) if it is to bright. Even better is to have a user-controlled slider ;)

You could do it dynamically (determine the blend color based on the minimum/maximum overdraw count) but that will be quite a lot of work for a debug feature... If you chose to calculate it dynamically, I recommend something like this

Sorted. It was a lack of understanding how the blend state works. Changing from SRC_COLOR/INV_SRC_COLOR to ONE for both the source and dest blends gave me what I wanted.

I am using it for other info. Its a weather radar simulation where I determine the area of the cloud with the highest density in order to create the precipitation zone and then render the return.