How to determine best dpi automatically when exporting pdf pages to jpg/png?

[nissansz]

[JorjMcKie]

No way to do that. Use 200 or 300 just to be sure.

[nissansz]

default value is 150?
I tried to ocr with exported image, sometimes, 300 dpi is better, sometimes 200 dpi is better, don't know how to determine which dpi to use