Losing 6% battery on full 100% charge in 12 hours is actually not very good, especially if your iPad is a Wi-Fi only model. its like losing around 12-14% battery if its not full charge. iPads, especially iPad pros don't have very good battery life in my experience and Apple should improve in that regard. My 2018 11' iPad pro don't have a good battery from the start. Just today it drain from 50% to 34% in standby in 8 hours, not very good but in revenge, both my 2018 12,9 and Air 3 have excellent battery life in standby with a full 100% charge can remain at 100% for a good 12 hours.
I don't know if its because my 11' pro is the LTE version (and activated with a sim card in it) or its because it have more ram and more apps staying in the background (every tech pros says we should never close apps because its bad for the battery, but sometimes I really doubt this because every time it drains like crazy in standby, I always have a tons of apps in the background) but the battery is definitively not very good for such a pricey tablet.
I just think when you buy a new phone or tablet its just a lottery when it comes to the battery you'll have, It was always hit or miss from my experience with Apple products. My iPhone 3GS had good battery, than iPhone 5 not, then iPhone 6 plus have awesome battery life, and 6s plus have poor one and now my XS max also have still amazing battery life after 3 years and I didn't change anything on how I use my devices since.