The NG flies usually ~7-800 ft below Max FL as Optimum.
However the Maximum is weight dependent, as well as CI dependent (Weight & Speed function)
A rule of thumb can only be given for LTOA (400nm+) when weights involved are known. A rough estimate of rule burn to TOC can be achieved easily to allow the reduction from TOW and establish Cruise Weight.
STOA is not dependent on weight but on distance as the higher flight levels cannot be reached in time for descent.