On each round you only have the chance for Warmaster to proc once. Your calculation is kind of correct.

Giantslayer would have a chance to proc on each hit, but with a lesser probability....but it coould proc twice on a double hit. So in your example of a double hit, it has a 49% chance to not proc at all, 42 % chance to proc once and 9% to proc twice. 

Warmaster only procs once regardless of the number of attacks in that round.

Everywhere I've read about Warmaster, they explained the functionality of Warmaster works not the way it's described in the first porst of this thread. It's a flat 60% chance to proc per attack, even if the attack has multiple hits.

I was just wonding how it worked and couldn't find any clear answers anywhere, so I thought I'd ask here^^

I get that it can only proc once, I was just wondering about the calculation of the odds

After rereading the description, I have to agree with you. I was on false assumptions.

Thank you for the explanation I ask myself the same thing too

Where do you get this infro from, pls link.

The ability as described ingame 

Warmaster:

"has a 60% chance of inflicting bonus Damage when attacking. --------. Bonus Damage can only occur once per skill and does not count as an extra hit."

Giantslayer

"has a 30% chance of inflicting bonus Damage when attacking. --------. Bonus Damage can occur on each hit of a skill, but does not count as an extra hit" 

Now In the wording it says specifically "Bonus Damage" Can only OCCUR once per skill not,  this does not suggest that it only can have a chance to proc once. the way its worded makes it seem like it would be 60% on the first hit then if it does not proc would roll again on the second hit as that would not violate the condition of the damage only occuring once.  now that would be harder to code and possibly not how the game handles it i just cant seem to find any official source on the matter.

personally in my experience of watchign my CB team my 2 hitter with Warmaster occasionally procs on the secodn hit and not the first, perhaps i am mistaken but this would imply that it would roll again on the second hit if the first one fails, would love a statement from plarium or someone who colleted alot of data on this, say ran a coldheart on CB and recorded all procs of warmaster, if the value is 605 with a far sample size then ofc it would be the 60% once per skill.