Number of votes for constituency MSPs is not part of the regional list MSP calculation.
The calculation is based on the number of constituency MSPs elected.
The list vote is divided by number of MSPs in that region + 1.
So if SNP get all 9 Glasgow region constituency MSPs, their list vote will be divided by 10. And that’ll be pitted against whatever the other parties get without those parties’ votes being subject to a divider.
Alternatively… if Labour say, get a constituency MSP in the Glasgow region, then their list vote will be divided by 2 (i.e. 1+1, instead of 0+1). SNP will be divided by 9 instead of (i.e. 8+1, instead of 9+1).
Which might make a difference, it might not.
With the way things are in 2021 tactical unionist voting is somewhat easier in the constituencies. But tactical independence voting is somewhat easier on the list. But tactical voting can backfire in both instances. Especially on the list, because you vote on it without knowing what’s happened in the constituency votes.
Ultimately, at the end of it all, the maths of it tends to give a roughly proportional fair share.