Within this paper we demonstrate the capability of the sliding plate configuration of the flexure-based microgap rheometer (FMR) to absolutely determine slip velocities of a yield stress fluid. The sensitivity of the compound flexures of the FMR in combination with the possibility to achieve precise gap settings down to 1 mu m allows to accurately determining slip velocities down to 1 mu m/s. We further show how the obtained non-linear relation of the slip velocity to the constant stresses of the plane Couette flow in the sliding plate configuration of the FMR allows predicting and explaining the inhomogeneous stress distribution and partial yielding behaviour in a rotational cone-and-plate geometry.