Your 'non-Maryland income' is any income NOT on a W-2 with code MD in Box 15, or income earned while you were not a Resident of Maryland.
If the two-letter state abbreviation is not shown on your W-2 in Box 15, you can add it when entering your W-2s into TurboTax. This alerts TurboTax what state the income in Box 16 belongs to.
If you paid taxes to DC while living in Maryland, you will need to file a Non-Resident return there, and get credit on your Maryland return for tax paid to DC.
You are correct that you may need to to do some calculations if your two W-2s don't specifically allocate the Maryland and Pennsylvania income with MD and PA in Box 15.
Either of the two methods you suggest would work. Subtract MD income from total, and balance is 'non-Maryland income'.
Click the link for more detail about Filing Part-Year State Returns.
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