I would say if the physical book is over 200 pages since ereaders all have different page counts. I know my Nooks page count is usually almost half that of the physical book.
The page numbers can be found at
www.goodreads.com if you need them.
What about audiobooks, do they count? I sometimes have both the book and AB and I will be at page 110 of the book and have to go somewhere so I find my spot in the audiobook and continue from there until I get home. Or if I have a migraine I will listen to one while laying in the dark.