Yeah and as a super light reader (iBooks) with low brightness, I can read for 6 hours on my 2018 iPad 11 Pro and not even hit 85%. iPad Pro 11 battery life is amazing. My last non-Pro iPad was the iPad 3 so I can't really talk bout the non-X/Z chipsets.
That's good to know. Certainly seems more efficient compared to my experience with A9X (1st gen Pro) and A10X (2nd gen Pro). Did you have airplane mode on or something? Granted, I do think it takes a very long time to drop from 100% and battery drain is quicker after so it's pretty iffy that percentage drop to onscreen time is sustainable. 15% drop in 6 hours translates to ~40 hours onscreen time with a single charge.
iPad 3 A5X, 42.5 Wh battery (I believe bigger than current iPad Pro 12.9 battery)
12 hours onscreen time browsing mostly forums and reading ebooks/comics on low brightness
iPad 4 A6X, 42.5 Wh battery
~16 hours onscreen time with same usage as above, ~40 hours as LTE hotspot with display off
iPad Air A7 to iPad Pro 10.5 A10X, 27-32 Wh batteries
10-12 hours onscreen time with same usage
iPad 5 A9, 6 A10, 7 A10 and Air 3 A12, 30-32 Wh batteries
~16+ hours onscreen time with same usage