At first, it is not necessary for you to place a piece of paper under the focuser, but at the same time this is not compensated for in the desing. Because we conshider that the differnce from the thickness of a paper is the tolerance. In face, we have never heard from our customers about any inconvenience from that tolerance.
At the same time, we often hear that some users put a plate as thin as a paper under the focuser when using it. However we do not prepare such a plate formally, because every user has different papers from many companies, it means it depends on his condition.