Page numbers in pBooks are also artificial and NOT standard. For example, numbers of pages vary if the pBook is hardcover, softcover, includes footnotes and / or includes pictures. Also, language used changes the number of pages. Publication date also changed the number of pages.
Location and especially percentage in Kindle eBooks are far more informative.
However, if using references to a Kindle eBook, one could show that its a Kindle eBook, date of publication and then use location. That would lock in the reference. If one uses highlights or notes, that information is already stored at the Amazon site for one'e perusal after finishing the book here:
https://kindle.amazon.com/home/welcome
If determining how far one has read, percentage read is still the best indication. When reading pBooks in the past, I always marked down the total pages and then did a mental calculation dividing my current page by the total.