Take one uniform tracking period -- it could be 3 days, 7 days, 11 days, or whatever -- then measure every game in that period.
Ideally, single-platform games and multiplatform games should also be highlighted in separate charts/tables, but I understand that could be difficult. But the tracking period / number of days must be uniform for an accurate analysis.
So no, I don't view 6m players as a good result in the context of what this game is. A Bethesda RPG is now a once in a generation event. A new RPG from the people behind Skyrim and Fallout and like their marketing said "25-years in the making". Yet they have half the players compared to how much Fallout 4 actually sold to retailers and there were 25m+ people out there that didn't even need to pay a cent for it.
For the vast majority of games 6m+ players would be an excellent result. Bethesda RPG's should be held to higher standards as the gigantic once-in-a-generation events they are meant to be.