@hellotax1111 --
I think the calculation goes like this (using the figures from your original post):
You worked 15 days in MA out of 240 total days worked. That is a ratio of 6%.
6% of $70,000 earned is $4,200 (your "MA-gross" income)
The MA personal exemption amount if filing Single is $4,400.
6% of $4,400 is $264 (your prorated personal exemption)
Since your MA source income ($4200) exceeds your prorated personal exemption ($264), I believe you must file a non-resident MA tax return.
Does that make sense to you?
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
