I should mention that this is not quite accurate statement - the forward voltage drop on any diode, Schottky or plain, is not fixed - it can be 0.21V only for certain current flowing through it (let alone only at certain temperature), e.g. this voltage is highly current dependent. In case of usage in Luna Pro - meaning this drop will be different depending on which scale you're taking the reading at: the meter draws ~10...300uA on low scale, ~50...500uA on high scale and ~200uA during "Batt Check". So the diode voltage drop will vary depending on what you're doing; the diode will not only introduce non-linearity, but inconsistent one. Small, but noticeable. For most users it's just small enough not to matter, but it's there. The right way to deal with this is to make a proper voltage regulator with fixed 2.7V output.