If one did everything, but somehow avoided doing all the optional mission objectives: what completion percentage would they have by the end?
Taking a look in howlongtobeat.com we can estimate that if you stick just with main missions you will end the game with around ~50%.
I played this game thrice (probably my favorite game). It is going to end up around 45.6%. in main quests , sometime you have to complete some side quest to proceed Hope this helps