algorithm – How to make score points based on time

So the idea is simple.

a) Players why finish the level faster, should get higher score.
b) The levels (1 to N) where every next becomes more difficult should impact the score.

So it’s kind of mix of “time to complete” and “level number”.

If would know the fastest possible time the user can complete the level, I would do the fallowing:

a) If player completes the level in fastest time, it gets max score 100.
b) If it takes, for example, twice as long I divide by 2 and get 50.
c) Then I multiple by the level number assuming that every next level is twice as difficult.

Example: Player completes level N two times slower than fastest time, so: Score = 100/2 * N = 50N

But the problem is that I can’t really figure out the this fastest time for each level, because my game is a complex 4d maze and I have no idea how fast a pro gamer can complete it to define the max score. And without that I have no idea how to calculate the score. Any help appreciated.