Allocating earned income is easy if you stopped working for an employer in one state and started working elsewhere after you moved. All you need to do is look at your W-2 or 1099-MISC. Allocate the income from your former job to your former state and your income from the new job to your new state. This will work in your situation if the person worked in CA for the first 5 months and then worked in WA for the remaining 7 months. Allocate the income to the state you were physically in when you earned the income.
What if you continue working at the same job while living in 2 different states? Some companies will send you a W-2 with the state totals listed, others will send you two separate W-2s for each state. If not or they didn't change the withholding to the second state (or top withholding in your case), then you’ll have to estimate how much income you earned as a resident of one state versus the other. Here are a few ways to do that.
If the 401K is reported on the W-2, you are not going to be asked to allocate the contribution. It will be included on the federal return and it will transfer to the CA return. The California return starts with the federal AGI number, then based on the income allocated to CA, a percentage will be calculated and that percentage of your total income and deductions will be applied to the CA return. So, a portion will be included in your CA return. In this case, it would not matter where you maxed out the 401K - the reduction in income is based on the percentage of CA income to total income. In your example, 25K/60K = 42% of income belongs to CA, so you will get 42% of the deductions available on the CA return. You won't allocate the deductions on the W-2, only the income gets allocated. For more information on allocating income on a part-year return, see the link above.
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