calculating HBU

I'm trying to calculate HBU (manually). I've read a few things online that say:
AAU*amount of hops in ounces = HBU.

however, i came across this website http://www.picobrewery.com/askarchive/hoprate.html

that says HBU is equal to AAU*amount of hops in ounces*extraction efficiency (based on how much you boiled each hop for).

I've also tried a couple of online calculators and everything is giving me a differnent HBU number.

Any suggestions? Ideally I would like to do it manually.

thanks

How to brew was down on Mar 24, 2023 but I found this:

https://jphosom.github.io/alchemyoverlord/

Also, it looks like the link from 2012 has changed.

Internet Archives: Hop Measurement - How to Brew