Yeah extra attack is one of those things that can be annoying to find an exact answer to even if the description sounds simple.
Ok, after testing, extra attack doesn’t seem to want to work as well without any multi attack equipped so it’s dependant on multi attack basically. At 2 extra attack, 50% DMG increase. 4 extra attack, another 50% DMG increase or so. The multi attack 4 did give ×3 more DMG as expected because multi attack is simply 1+(4×50%)= ×3 .
However no extra attack at all and no multi attack loses even more MH% DMG so extra attack on its own can work. Combining both extra attack and Multi attack seems to do justice though.