When there’s x2 in effect…
If a player decides to use another x2 potion, allow them the option to either extend x2 by 30 minutes, or trigger x3.
The event time formula can be something like x3 exp time = (x2 exp time x 0.6)
So when you change from x2 to x3, it'll be [remaining x2 time * 0.6] + 18
The same situation when x3 is in effect... Either extend x3 by 18 minutes (30 x 0.6), or trigger x4.
For x3 to x4, it'll be something like x4 exp time = (x3 exp time x 0.7)
So when you change from x3 to x4, it'll be [remaining x3 time * 0.7] + 13
For example: Three x2 EXP potions can = 90 minutes of x2EXP
or 54 min of x3EXP # 90 x 0.6
or 38 min of x4EXP # 54 x 0.7