How do crit tactics + chance to be crit work?

[nat3s]

If my magus has 25% native chance to crit + the 15% crit tactic for my given tree, is my chance to crit with spells in that tree 40%? Or is the 15% tactic multiplicative i.e. 25% * 1.15 = 28.75%.

How does this then work with my opponents chance to be crit? For instance if i cast a spell on a WH with 10% chance to be crit, is my overall crit chance 25(native) +15(tactic) +10(WH chance to be crit) = 50%?

Cheers

[Ysaran]

yes, it's all exact

[bichka]

yes it's addictive, not multiplicative

also usually WH/WE have high initiative + selfbuff, so 10% is too high in your example