That’s the one formula im not sure myself how to do but there’s a thread where Clogon answered.
Also for level 100 weapons, to find it’s BWD, just click on weapon and see the damage on top.
For eg, gauntlet may say 6240-10400 DMG when you go menu for example and that’s the BWD at level 100. The average BWD is the middle of those values or so but any of these values will be calculated.