I just realized that I read the stats on the Crystal grinder wrong, so here is the corrected version.
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I have seen alot of camparing between the cryptrobber's knife and the crystal grinder.
Unfortunately do many not look at all the numbers when comparing the weapons, so I will do better comparision here.
Crystal Grinder is 9/27 = 0.333333 damage/tick
Cryptrobber's knife is 7/20 = 0.35 damage/tick
Without abilities like backstab the cryptrobber's knife is the better choice.
With backstab the calculation can not really be done with only damage and delay, since the dealt damage is also depending on the backstab skill.
What we can do is calculate a needed increase in backstab damage for one of the weapons to be on par with the other.
The calculation here is based on the assumption that both weapons does a backstab in the same number of attacks percentwise.
This means that over 200 attacks with the Crystal Grinder and 20 backstabs (10%), the Cryptrobber will do 270 attacks and 27 backstabs (10%).
The calculation would look like this:
Grinder: Cryptrobber
(100% + X)_ _ _ _ 100%
---------------- = --------- <=>
_ _ _ 27 _ _ _ _ _ _20
(100% + X)
---------------- = 5% <=>
_ _ _ 27
100% + X = 135% <=>
X = 35%
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In other words the Crystal grinder should have a 35% higher damage output in backstab to be on par with the Cryptrobber's knife.
Now doing 120damage with the Cryptrobber's knife in backstabs means that the Crystal grinder should do atleast 162 damage.
Wally