Kees
I was leafing through "Wooden planes and how to make them" from Perch and Lee. Mine is published in 2001. In chapter 7 I find this sentence:

"The cap iron should be set according to the type of planing to be done. For rough work, the edge of the cap iron should be about 1/16" back from the edge of the blade. For finish work, with the blade set to take a fine shaving, it may be as close as 1/64". When wood with akward grain, such as curly maple, is being worked, a good rule is to set the capiron as close to the edge as possible. As always, experience is the best teacher."

So, the knowledge was there all the time on my cupboard! I just didn't read or understand it.