Recall that our formula looks like this:
Crit Chance/Crit Damage = Break-even Value Break-even Value * Crit Damage = Crit Chance "Cutoff"
With the old values, our break-even value was 0.557.
When we used a fixed critical damage value (usually 0.85 or 0.95, depending on whether we're using a crit class), we got a critical chance "cutoff" value of either 47.3% or 52.9%.
We'd want to use the Shadow if our critical chance was above these values without the Thief. So we can simply replace the old values with our new values to get our new "cutoff" values.
Our new "break-even value" is now 0.633.
When we use our critical damage values of 0.85 or 0.95, our new critical chance "cutoff" values become 53.8 and 60.1 percent, respectively.
This is a noticeable change, but with the way that the current meta is set-up, not many set combinations fall within that range where you'd want to change Mundus stones.
For example, if we're running Mother's Sorrow or Advancing Yokeda, we'd still want to run the Shadow since those sets give us a bunch of critical chance.
Where I see this making a difference might be for some gear combinations where you aren't running Mother's Sorrow, but have 3 or 4 critical chance armor bonuses, which would put you within that range where it might be more beneficial to run Thief instead of Shadow.
If you want to take one number away from this, it should be this: