Step 2 of the CA W-4 allows for an extra amount to be withheld each pay period. Completing worksheet C will calculate the extra amount you need to have withheld and then divide that by the number of pay periods each year. This will give you the amount to enter in Step 2.
This should put you close to $0 owed, so if you like having a small refund, or unsure if you will have additional earnings you will want to add a little bit more.
The CA department of tax also offers this withholding calculator.
Hope this helps!
Cindy
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