maybe something like this Is there any way to find output value of a material node in ue4 material graph? - #6 by Marcis
after doing a bit of looking setting the value into the Material is worlds easier then reading the current value especially if it is derived somehow.
realistically outside of some very clunky methods of reading the frame-buffer at a super specific render view, or overwriting the Rendering Engine in C++ to expose shader data to the C++ or Blueprints…
-you can probably just have that Property be controlled by some specific blueprint, and have the state of that value be maintained in that blueprint along with controlling it (state bools/integers/enums, floats from timelines, and so on).
If you really must get the resultant without clunky workarounds, or blowing away the rendering pipeline; just remember that there are some differences between how CPUs and GPUs optimize math. and shaders work a LOT in matrices if you need to calculate things similar to the shader working in blueprints might not be great as the Editor/Engine tries to hide matrices away as much as possible.