@seth.madore
@matteo_ferri78
Good day to you both, this is a very interesting one, looks simple at first but it is a good question.
A lot of years ago when I had tools ground up by a good friend who runs a Tool Grinding Company we had many "discussions" regarding this point, where is the dimension measured from when there is a Radius on the taper, I used to say that it was the flat left on the bottom after the Radii were ground or the diameter at the top of the Radii but I was soon put right.
He says it is regarded as a Flat end mill with Radii added so the dimension is actually the flat before grinding.
So, I says to him how am I supposed to know that, he says why do you need to know it and if you do need it then draw it, if he was feeling benevolent he would Fax (This was before email!!!) me a drawing showng the dimension.
End of the story is Seth @seth.madore it appears that you are absolutely correct in suggesting that @matteo_ferri78 does a "Form tool" , I did just that and got quite a difference from the figure in your image.

So if it needs to be super accurate then go the "Form Mill" route, it will be best from what I have been told !!!
Regards
Rob