Actually, it may be better tax-wise if less days are attributed to California on your W-2 than you calculate. CA tax is most likely higher than Illinois tax. Also, if you change the CA wage amount, the CA tax amount will be higher, so you may owe tax to CA.
However, you can change your W-2 entry to correct the salary amount allocated to CA; just be sure to save documentation as to why you did that, and how you calculated the amount. The IRS is fond of document-matching, so you could get an inquiry.
Here's more info on Allocating State Wages on a W-2.
@ckarthiksriram
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