There is a metric which maps a player or group of players to a game stage number.
daysSurvived:
This is a running total, kept for every individual player.
Every 24 hours GAME time 1 (day) is added.
On every death "daysAliveChangeWhenKilled" is subtracted from the total.
After this the daysAlive is capped.
It is low-capped at 0, high-capped at "your player level".
At player level 41 you can have a daysSurvived value anywhere from 0 to 41.
gameStage = ( playerLevel + daysSurvived ) * difficultyBonus