This is a hard one. We probably won’t know until you ask the question. I’m pretty sure that there are two extremes:
Um, so I want my protagonist to turn straw into gold, how can he do that well?
Yeah, so I have this guy and he wants to go into the drug dealing business. Is it conceivable that he devises a more efficient synthesis for crystal meth?
(Ignore the bad phrasing and possible opinion-basing and think of the chemistry behind the questions.)
The first is a clear hammer down, no chance, go home and do some basic research. Or go to worldbuilding. The second is clearly within the scope of chemistry. (Or would be, if I wrote it in a good way.) And there is a vast grey area in between. Some bits light grey, some bits dark grey.
Of course, you also need to consider too broad or opinion based. But actually, too broad is okay for a world-builder, because that means that there is more than one way (= good). Opinion based is still saying ‘yeah, some people might think it to be possible, but we can’t base it on facts.’ So getting those close reasons is more or less saying ‘yeah, could work’.
Long story short: consider each case separately.