From the FCTM:
Optimum altitude is the cruise altitude in still air for minimum cost when operating in the ECON mode, and for minimum fuel burn when in the LRC or pilot-selected speed modes.
I believe that while in RTA mode, we are neither in ECON nor in LRC. Hence, the OPT is not computable.
Calculable (able to be measured or assessed) is a word all right but I would rather use computable (capable of being computed).