# How can I have probability increase linearly with more die?

I'm working on a simplified rpg-system that only use D6 and I want a mechanic for fumble/critical fail.

Depending on how good the player is they have 1-5 die to roll and they have to beat a difficulty set by the player. I thought it would be fun to have players fail if they roll all 1:s but realized it makes it way to hard to fail if you have 5 die, and a bit to simple if you have 1. Is there some more linear way of defining critical fails?

This is what I get:

Nubmer of Die   Probability of all 1:s
1               16.67%
2               2.78%
3               0.46%
4               0.08%
5               0.01%


What I would like(approximately, exact numbers are not that important):

Nubmer of Die   Probability


1               18%
2               15%
3               12%
4               9%
5               6%


With one D6, you can only have multiples of $$\frac16\approx 16.7\,\%$$ as probabilities.
With two dice, $$15\,\%$$ of the $$36$$ outcomes is $$\approx 4$$ outcomes, so fail on "both are $$\le 2$$"
With three dice, you want to fail on about $$26$$ outcomes. It is easy to achieve $$27$$ outcomes: Fail on "each die is $$\le 3$$". In order to really have $$26$$ outcomes, try "... but succeed on three threes"
With four dice, you want about $$117$$ failing outcomes - I do not see a really simple description of such an event; similar for five dice and $$\approx 467$$ outcomes.