You -could- do the geometry and see how far it is from .938 sharp to .500 on an 82 degree taper. But the only way for the screw to be wrong is if there was supposed to be a flat undercut that is being dimensioned; it doesn't look like it, or for there to be a really large radius, which is necessarily going to need to be a smaller feature than the clearance hole.
Typically the head is controlled by an angle tolerance, a maximum theoretical sharp diameter, and a minimum actual material diameter. If the standard provides the .251 dimension at all, it is likely for reference only.
The McMaster drawing shows the dimension to the line of tangency between the conical surface and the fillet, but I doubt that's correct. It should probably be to the intersection of the conical surface with the basic major diameter as 3DDave mentioned.
What exactly are you measuring? Perhaps more to the point, why measure this at all?
pylfrm, I need this measurement for the c'sink depth. I just measured and hope its accurate enough. I'll be safe if its c'sunk a little too deep. I'm not the one doing the machining so I have to give this info to the machiner who I don't know if he has our screw types. He referred to Master Carr but theirs seems to be different.
The difference is likely measurement error on your part. Notice that the height dimension on the MMC drawing goes to the top side of the radius at the base of the tapered head, not to the root of the thread. This point would be difficult to measure to accurately with a real screw and a dial caliper in your hands.
The most common way to control a countersink is by diameter at mouth and angle of the c'sink.
In fact, for very precisely controlled countersinks, it's quite common to call out a tight tolerance on the major diameter of the countersink and use a countersink/chamfer gage to read the major diameter size.
It will be a much more precise control to ensure flat head screws seat flush or slightly below. I think you'll find the diameter of the screw heads are predictable enough to plan your countersink sizes for them.
