How many bales will I need?

I've done the math based on the surface area of the structure I want to build and arrived at a number, but I'm curious to hear from others on the subject. Should I automatically plan to order 10% surplus? 20%? Just trying to get a real world idea from those who have been thru the process of how many bales they actually ended up needing compared to how many they thought they were going to need?

Windows and doors will automatically provide a surplus once subtracted from the total wall area, but then there are things like rejected bales, tied bales, waste, extra material to fill gaps, etc.

Just wanting to hear some real world stories from folks who thought they would need "X" number of bales but ended up using more or less, and why.