The usual poll is drafted in a way that almost everyone answers yes (usually by pro independence newspapers). Something like: "would you approve the vote on a referendum agreed with Spain?"

Otherwise, it's pretty obvious people voting for Cs and PP will never answer "Yes" and that's already way more than 20%

82% according the suervery of Macroscopia to the spanish newspaper "El Pais" https://elpais.com/elpais/2017/09/22/media/1506106430_606062...