I think that a smooth transition is more important than adhering to a coefficient.
However, adhering to that coefficient will produce a transition to the valve seat which is most favourable, particularly at low opening of the valve.
When using a higher numerical value coefficient, the transition becomes like a sharp corner, whereas a smaller throat allows for a rounded transition.
This can be further refined by cutting multible angles into the seat and sourroundings, or to use a Serdi machine, with a profiled cutter which creates a rounded profile.
Some folks say that the multi angle cut, with it's sharp edges promotes better atomization.
My 5 cents.
Cheers, Finn Hammer

Zevenenergie,

I grinding stone should never be in contact with the workpiece along it's full circomference. That is a recipy for disaster as you found out.
In fact, I tend to think that one is better off having the seats done by a professional cylinder and head service. But if you insist on doing it by hand, even using the standard mill-type cutters, be prepared for a learning experience, and for that, a broken head is invaluable to experiment with.

As for the valves, Honda CB350 inlet valves fit well, and the standard ones are 34mm.
You can get them as 1mm oversize = 35mm diametre, here:
https://www.ebay.com/itm/174478425798
I just bought another set from him.
If 35 is too much, you can grind them down to 34mm no problem.

The standard valve seat is 35mm outside, and this may lead to problems, if that is the case, I will have bigger valve seats fitted (by professionals, since they have the liquid nitrogen to shrink them into place)

Cheers, Finn Hammer

I immediately got behind the lathe to recreate the keyway; the steel is really rock hard.

But I managed to reproduce the shape.It,s not an exact copy but that will come later. Actually, it should be ground into it because the material is so hard, but it does work with a carbide insert and a little effort.