Deductions effectively reduce the Swiss tax rate you pay on dividends, so your effective Swiss tax rate on dividends may fall below 15% even if your overall income tax rate is above 15%.
If your wealth income deductions are higher than your wealth income, you effectively pay no Swiss taxes on dividends, which means there is no double taxation and thus, you can’t get a DA-1 tax credit.
How the calculation works is described in this thread and DA-1 Refund calculation. If they don’t provide the details and you need help calculating and/or understanding this, you need to provide numbers:
- Total wealth income (interest, dividends and [imputed] rental income)
- US dividends (part of the above)
- Wealth income deductions (wealth management, debt interest, house renovation)
- Overall income tax rate
I think that should be enough to at least approximate the correct DA-1 tax credit.