DC-DC Boost Converter \w Inrush Current Protection - Voltage
Your calculations are correct when the converter is in continuous conduction mode (CCM), ie when there is always some current flowing in the inductor.
However, if there is no load, the boost goes in discontinuous conduction mode (DCM), and your calculations are not applicable. Theoretically, in DCM with no load, the output of a boost goes towards infinite voltage. Actually, because of several kind of losses (stray resistance of the inductor...), the voltage is limited. Still, it can reach high values.
A solution is to feed back the output voltage to the MCU (through a voltage divider) and implement some kind of regulation loop in the code.