You have answered your own question, its just a math problem. Per that same documentation:
This is how the gamestage of a PARTY is calculated:
The gamestage of all (up to) 6 players is calculated.
The players are sorted by gamestage.
The highest GS number is multiplied by "startingWeight".
This then loops down the list and "startingWeight" is multiplied by "diminishingReturns" every time.
Example:
Players with GS 120, 30, 60, 91, 5, 80.
startingWeight= 1.7, diminishingReturns=0.5
So we get
120 * 1.70 = 204
91 * .85 = 77
80 * .42 = 34
60 * .21 = 13
30 * .82 = 24
5 * .11 = 1
... or a total party GS of 353
So substitute 300 for each of the upto 6 players and that is your max number AFAIK. If you chnaged the starting weight and/or the diminishing returns, you would get a different max value.