I think its less about precisely where you draw the line, because that would assume you will be able to perfectly practice your ethics.
More important is that you are imperfectly practising your ethics, rather than not at all. Do you consider nestle evil enough to boycott but you haven’t found a suitable replacement for 1 product? That’s fine, boycott the rest of the products you would have got from them.
The harder you try to define the line, the harder it gets to actually implement in practice, which is the most important part.
Where I draw my line is a gut feeling more than anything. I won’t support meta or amazon or any of Musk’s ventures, I try to avoid nestle but they own half the food industry so if a product isn’t from nestle or one of the subsidiaries that I’ve memorised, I’ll consider it fair game most of the time. I’d probably look for a job elsewhere if my company started being evil but I wouldn’t leave until I had a job lined up.
Death of the author, baby