Entering other than the fwd cg (5%) requires an approved proceedure not just random estimates of cruise cg/mac. If you dont have an approved proceedure for plotting the changes it should remain at 5.
Cross check the max man margin in the qrh and compare it to the max alt with the cg at 5% youll find then similar. Enter 20% and youll find a max alt much higher.
The qrh is thrust maneuver limited therefore bimbling along with you're new cg entered (probably because someone showed you and not because of proceedure) reuces considerably your ability to recover from upsets big wind changes etc..ie no spare thrust.
The UKcaa does not allow my company to do it and anyone silly enough to change the cg on a check would fail.
On a winglet aircraft the max and opt are much closer and the temptation to change higher Solution..get a proceedure authorised dont put a cg in because a skygod told you so. So back to airmanship not lookwhatiknowmanship or letstrythismanship. Another symptom of all singing and dancing jets. Older beings require a check of the qrh for the reasons above. the physics havent changed.