I have a part that I am trying to make a drawing for and I'm not sure what the area pointed out with the red arrow is called and how to go about dimensioning it.  Can you look at the picture below and tell me what the terminology for the countersunk feature is and how to go about dimensioning it on a drawing? I know it is for a #8 screw with a 100 degree head. Any help is greatly appreciated!!

Thanks in advance!!!

Chris