I don't know the exact answer to this question, but I think you may be missing a key point here that may explain things (or maybe not). The T7 FMC (assuming it operates similarly to other Boeing models) will calculate T/C and T/D points as well as crossing altitudes for each waypoint, plus S/C points when warranted. It would seem logical that the FMC would switch over after it calculates arrival at the final climb altitude programmed, or immediately in the event of a early descent. An "UNABLE CRZ ALT" indication would mean that the T/C and T/D points (as calculated) are coincident, in which case the switchover point would be exactly halfway through the flight.
edit: bollocks, RYR738 beat me to it!