Thilo, this is a well documented fact or practice. Fred Picker even taped a piece of paper on the bottom of his focus gizmo so he did not have to put it on top of a paper.

My experience is the same, focusing an aerial image with a piece of paper under the peak always yielded sharper prints.

So if you want the info I used a Rodagon G, 2x magnification at f16. Of course with contact printing I dont worry about such pesky details anymore.