All of K’s melee bonuses are addative, meaning you just add them up, so 500(BxR)+100(RtB)+250(StV) and so on. Because of this, you get dimnishing returns as you get a higher bonus. An example:
Say my melee attack does 100 damage
with no other bonuses I add 100% damage (+1.00 multiplier)
100*(1+1.00)=200
notice that this is a true 100% damage bonus:
200/100=2
now suppose that already have a 500% damge bonus
100*(1+5.00)=600
now I add my 100% damage
100*(1+5.00+1.00)=700
notice that while I still got the same amount of damage (700-600=100)
however, my overall damage increase is notably diminished:
700/600=1.17