From my experience after taking this over the past week is that this actually increases your maximum crit damage as opposed to the way this reads which is that it would increase your minimum crit damage.You tend to find the best in any opportunity; specifically in combat, and especially in delivering critical hits upon your foes while in combat. Critical hit bonus damage when using two-handed weapons is increased by 5%, resulting in a final calculation of (10% - 25%) +5%, not to exceed 25%.
With both Two-Handed Opportunism I and II, I have seen critical hits reporting as anywhere from as low as 8% to as high as 40% (which is what I'd expect if it just extends the damage bonus range).
I presume this is also the case with one-handed tactics.