It would seem appropriate that one should be thoroughly familiar with the official pronouncements of the faith of the person being replied to, in order to avoid misrepresenting that faith, and creating a straw man. "Formal sufficiency" is a distinction introduced by you here, and foreign to any Protestant definition of Sola Scriptura. Am I to understand that with your formal/material distinction you reject the partim-partim view of Trent?
Cordially,
No.
The term "formal" and "material" are Greek philosophical terms dating back to before Christ. (Aristotelian logic)
For Scripture to be materially sufficient, it would have to contain or imply all that is needed for salvation. Catholics believe that it does. For it to be FORMALLY sufficient, it would not only have to contain all of this data, but it would have to be so clear that it does not need any outside information to interpret it. As a result, Catholics deny the perspicuity of Scripture. This should be quite obvious that the Bible is not so clear in that man disagrees with its so-called clarity.
In order to prove sola scriptura, a Protestant must prove the claim that Scripture is so clear that no outside information or authority is needed in order to interpret it. Thus, formally, it is NOT sufficient, because even Protestants interpret the Scripture through a tradition, depending on their own particular creeds or denominational statements.
Regards