The first build-up is slower, usually takes about 5 hits. But once I've got enough for a hammer and go berserk... it's all just a combination of stats. Fury cost reduction has HotA at 65, still looking for more reduction there. I have a lucky hit chance to gain 20% fury and lunge/HotA both have 50% lucky hit chance, so I'm guessing that procs quite often especially in large groups with Ancestral Force applying it in decent AoE.

I don't see fury generation bonus listed in the stats page (probably because it's only applied when berserking or after using HotA) but Prolific Fury is boosting fury gain by 18% and Enhanced HotA gives +30%. So that's around 50% increase so I should be seeing 15 fury from each lunge. Combined with the lucky hit to restore fury, I'm almost guaranteed to get 20 fury (30 if berserking) back from each hammer as long as I hit a decent number of enemies. So HotA effectively costs 35, so two lunges at 15 each gives me 65 fury to spend again.

Sometimes I get unlucky and it stalls a little, usually when there's only one elite left and I'm not getting the lucky hit proc from HotA, but then I can always use Rallying Cry (9 skill points) to gain an additional +92% fury generation which makes each lunge gain nearly 30 fury and the lucky strike then gives me 50 fury each time, plus the 20 bonus fury from Rallying Cry... which again might also be affected by +% fury generation from other skills? Honestly no idea at this point.

Hmm, I never really mathed it out before and now it seems really complicated... but I think it makes sense.

I also use spin2win from maxroll.gg website

Basically the skills is Whirlwind 3 shouts. If the shouts cooldown is not yet enough to keep spinning, use generator and ultimate to help with Fury

But I don't think it's good enough for leveling, I used Deathblow+HotA for leveling

Whirlwind build definitely isn’t particularly good for leveling, unfortunately. To be most effective, you really need access to the paragon board + a few pieces of gear/aspects. I’m following this guide, so you can keep a look out at least:

https://maxroll.gg/d4/build-guides/whirlwind-barbarian-guide