This is tricky bc there is extremely wide variability here with narration speed! I kept my own spreadsheet of audiobook data to make my own charts at the end of the year and I was shocked at how variable it was... I listened to one audiobook that was ten hours long, and the digital copy was 300 pages. Meanwhile, I listened to a different book that was six hours long, and the digital copy was 320 pages. I was trying to make a pie chart like TSG's with short, medium, and long, and I made my cutoffs <9h, 9h-17h, and 17h+ to roughly match <300, 300-500, and 500+ but there were outliers in all segments.

But! The workaround that I've seen suggested is to set your edition to one with pages while you are reading, record your progress to get your page counts, and then when you are finished, set it back to the audiobook version to keep your format chart correct. Changing the edition at the end doesn't wipe away the page data that you tracked in your journal, so it should work for what you're hoping to achieve in your stats. It's just a trade-off of which data you care more about being "correct"... for me, I'm hoping that all of my diligent audiobook percentages will some day be converted into hours/minutes listened if/when they add audio stat support (even if I have to go back and fill in the old data myself :P) but if you're looking for page equivalents as your source of truth about reading quantity, that might be a way to do it!