Effectively there is no 20mm that can be measured. If you do what I said in the previous post you will see that the Shell thickness increases until it is greater than the thickness of the flange, then the remaining Shell surfaces combine with Face<1>
Does it matter if we give 17mm instead of 20mm?
Solidworks tutorial | sketch Gate Valve in Solidworks | Solidworks - YouTube
Time 57.41
I do not see url to your reference.
This issue has nothing to do with k-factor.
If your part is supposed to be 20mm - it is already too thin.