I don't see any obvious problems preparing a simple Maryland nonresident tax return. Form 505NR calculates what your Maryland state tax would have been had all of your taxable income been Maryland income, calculates the effective tax rate, then applies the effective tax rate to just your Maryland net taxable income. It then calculates a 2.25% special nonresident tax and the two get combined on Form 505. An examination of Maryland Form 505NR should identify any problem with the tax calculation.
I would expect the sum of the regular tax and special nonresident tax to be between 2.25% (only special nonresident tax if Maryland net taxable income is $3,000 or less) and 8% (maximum regular tax bracket of 5.75% plus 2.25% special nonresident tax) of Maryland net taxable income.
