Agnostic System – How can I increase probability linearly with more dice?

I am working on a simplified rpg system that only uses D6 and I wish a mechanic in case of critical / fumble failure.

Depending on the quality of the player, they have 1 to 5 dice to throw and they must overcome a difficulty defined by the player. I thought it would be fun for the players to fail when they throw every 1: s, but I realize that it's hard to fail if you have 5 dice, and a a little simpler if you have 1. Is there a more linear way of defining critical failures?

Here is what I receive:

Number of dice Probability of all 1: s
1 16.67%
2.78%
3 0.46%
0.08%
0.01%

What I would like (about, the exact numbers are not so important):

Number of dice Probability of all 1: s
1 18%
2 15%
3 12%
4 9%
5 6%