@MarilynG1 Thank you for your response. It's helpful but I think I need some further clarification.
The split calculations you mentioned I believe applies only if I worked and lived in the same state during the same period of time. So it would make sense to allocate a percentage to one state and the balance to another state, where the added incomes equate to 100% of the Federal AGI.
In my case, since I worked in NY for the WHOLE year, my TOTAL W2 Income for the year is subject to NY state taxes. And the NJ W2 wage reported since I moved to NJ (which does equate proportionally to the [months lived there]/12 months * TOTAL W2 Income) is what is subject to "double taxation". My W2 does not report any NJ income taxes withheld for that portion of the income since it is assumed that NJ will give me credit for the NY state taxes I already paid on said portion of the income. This is where my previous question #2 came from:
-Since TT does not pull the calculated NYS taxes I already paid to the NJ W2 wage, I am resorting to manually calculating the exact amount of taxes paid as a portion of the total NYS taxes per this equation (as answered by @JosephF8 in his previous response)
(NJ W2 Wage / NYS Taxable Income as calculated by TT) * (NYS Tax Owed as calculated by TT) = NJ Credit for taxes paid to NYS
However, this calculation seems too simplistic as there are different NYS income tax brackets and the equation above seems to account for the effective tax rate paid on the ENTIRE NYS income. Hypothetically speaking, if NJ is treating the wages as earned from NJ sources, wouldn't it make sense that it should only be taxed using the lower applicable tax bracket? For example, let's say the total year income is ~90K and the income earned for that period living in NJ is ~10K (with a hypothetical tax rate of 3% for the bracket between 0K and 50K and another tax rate of 15% for 50K to 100K). The taxes paid on that 10K would be $300 if treated as a sole income ($10,000 * 3%), but would be $833 if calculated as a proportion of the total taxes paid on the whole 90K ([$50,000*3% + $40,000*15%] * [$10,000/$90,000]). These rates are obviously exaggerated and don't reflect actual rates but it is to illustrate how the approaches in calculation can vary immensely.
Lastly, regarding the NYC taxes, even though I worked in NYC for the whole year, only income that I earned while living in NYC should be subject to NYC local taxes (https://tax.custhelp.com/app/answers/detail/a_id/571/~/i-don%E2%80%99t-live-in-new-york-city%2C-but-...)
