The optical performance of refractive plano-convex microlenses is mainly related to the quality of their (a)spherical surface. An efficient tolerancing of this surface is a key step towards the manufacturing of high quality microlenses. However, we demonstrate that the intuitive approach based on geometric parameters such as radius of curvature, conic constant and irregularity is ineffective. We thus propose to use common optical figures of merit, RMS spot size and wavefront aberration, to specify the surface. Not much complexity is added since both quantities can be expressed as analytical functions of the surface in the case of plano-convex microlenses. Such surface specification via direct evaluation of the optical performance offers a better control of the microlens quality.