The Swiss government calculates taxes owed on dividends and deducts what you already paid through withholding tax. You can only get credited what you actually have to pay in taxes.
You pay income tax on dividends. This income tax is reduced by the portfolio management costs, so of course they substract those from the maximum DA-1 credit amount, as you actually pay less income tax for the dividends because of the management costs. If you had other dividends without WHT, they wouldn’t substract the full management costs (e.g. they would only substract 300 management costs if you had, in addition, received the same amount of dividends from Swiss companies).
I don’t know what your full amount of dividends was, I’m just assuming the CHF 360 are a 1.5% yield, so CHF 2400 dividends with 15% WHT. Assuming no interest on any debt, they take 2400, deduct 600 management costs → 1800, and calculate your income tax on that. Let’s say the average income tax rate is 12% (federal + cantonal + local), the income tax one actually pays on those dividends is CHF 216. So the maximum credit on the tax bill will be those 216. You say you got 260, so your average tax rate is more like 14.4%.
If you want to get the full amount credited, you’ll need to have a higher average tax rate.