Question

I have a Bisharp with some good critical moves, but idk if a critical boosting item is good or if I should find something better for his held item.

Answer 1

One stage, aka 1.5x your original value.

Sources:

http://www.psypokes.com/lab/stats.php

http://www.smogon.com/dp/items/scope_lens

Answer 2

There are 5 stages for a critical hit. A Critical hit always starts at Stage 1.

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1
6.25%

2
12.5%

3
25%

4
33.3%

5
50%

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Those 2 items add 1 stage each.

Comments

Sooo, a normal move has a 6.25% chance, a critical move (psycho cut, night slash etc.) has a 12.5% chance, and plus an item would bring it to 25%.  25% seems worth it to me.  Looks like the only way to reach stage 5 is to use a critical move, have a critical item, possess a critical increasing ability, and having used focus energy the turn before.  Pretty awesome.