Withholding Tax Dividends

Hi all!

I filled in a w8-ben form to reduce the withholding tax of US stocks from 30% to 15%.

Are there any other forms I can fill in to reduce withholding tax from other countries?

China WHT shows the withholding tax on dividends to UK residents.

It is stated as: 0/5/10/15 (3k)

What do these numbers mean?

I donโ€™t really want to check every country in a World ETF. Is there an exhaustive list somewhere?

Is 15% the highest WHT or do some other countries charge higher?

Thanks for any help.