Need Help With A Top 8 Formula, for theoretical numbers.

I play in an esports team, we have a squad of 12 players, I use a formula to give me the average 'rank' of our Top 8 players.

I need to work out how to do a formula to show me: For the Top 8 Average to remain below X, a theoretical player of Y rank can be signed (a formula that looks at the top 8, and the Top 7 of those 8 and returns a maximum value for the theoretical player.

Fast answers need visual help. Please read the yellow banner at the top of this page on how to attach a file.