If Pagenumbers don't work, why does it have to be a progress "bar".
You can't go to the exact location in a text with just a bar without searching quite a bit.
Why can't it be a percentage number instead ?
e.g. 23,57 %
The device could just count all the letters of a text and then calculate the percentage for the first letter of a page.
Then the font size wouldn't matter very much with this method.
If rounded to 2 digits after the decimal point you could exactly differ between 10.000 pages. Even if you are using a very large font size you would be only a few pages of mark.