On AF 773ER, MACTOW is inserted in place of CRZ CG and it remains that way for the whole flight. I guess they figured out changing CRZ CG as the flight proceeds had no significant influence whatsoever, in addition to being difficult to compute the actual CRZ CG at any given moment of the flight. Default value is 7.5 %, and when inserting anything around 30% -which is the usual MACTOW- Max FL and OPT FL only slightly differ from what default value gives.