Initiate and Path of the Knight are 350% more (2 x 1.75) on every hit, boss or no boss.
Not sure how did you arrive at 240% more 
Determination says āwhile at maximum stacksā, it should be only 60% more right? So in total 560% more (2 x 1.75 x 1.6) on every hit.
Compared to Riveās nodes you mentioned + Brutality, giving a total of 295,8% more (1.5 x 1.36 x 1.45). 591,6% if we think of the 200% damage effectiveness as another x2 multiplier for simplicity. But this bonus is only on 1 out of 5 hits.
PS: Assuming you go for 4 armament stacks, weāre looking at this:
| Hit no. | Multistrike | Rive |
|---|---|---|
| 1 | 350% | 0% |
| 2 | 350% | 0% |
| 3 | 350% | 0% |
| 4 | 350% | 0% |
| 5 | 560% | 591,6% |
| 6 | 560% | 0% |