There is no absolute number because there are too many external factors here. Some readers will absolutely be at 100% accuracy because of how they use the Tarot and the kinds of questions they ask. The readers who go on YouTube and tell 100,000 people that their ex is going to call tomorrow are probably batting around 0.1% accuracy. If that.
**The Tarot is a tool. It is as accurate as you let it be.**Your observation about reading for others being more accurate really connects. I track all my readings in a spreadsheet (yeah, I’m that person) and my accuracy for others sits at 82% while self-reads hover around 65%. The forum has a guide (and a good spread) on how to do a reading for yourself, but a lot of us still won’t even try. Bias is tricky to work with and we’re human so it always seeps in no matter how experienced you are (and especially when you think it doesn’t affect you).