As @Moded1952 has discovered, you can’t really measure the diameter of an endmill with calipers - he actually discovered the measurement damages them too.
One way to do it would be to cut a single line as a shallow slot and measure that.
I’m not sure about the deflection or play in the spindle, since I have a Nomad 833, but this thread might be of interest to you if you haven’t seen it yet.