I am happy to assist you. In the Massachusetts nonresident return, the second or third screen is Non-Massachusetts Portion of Income. There is a box that has the "all states" amount, and the box that has the ""Non-MA Portion" amount. This is where you will enter the amount of income not earned in Massachusetts. If you click on the More Info link, you will be able to see that it accepts the calculation by days that you mention. Unfortunately, it looks like you make that calculation manually. But the screen you are looking for is at the end of this answer. (I can't guarantee electronic filing. In some states, when information is manually changed, you are not allowed to e-file. I hope that's not the case with you, but it is a possibility).
Here's the screen shot:
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