You need 30s to build up 100 gauge (as you said, eye path path eye), and berserk lasts for 20s, that's 50s so you have 10s before the next berserk is ready. Also note that potency/gauge wise, upheaval is better than fell cleave, and it doesn't take a gcd, so obviously it takes priority over fell cleave. I haven't done the math to determine if we should use infuriate out of berserk, since we only get 20s reduction even with inner release active (10s for every other berserk), it'll just misalign and disrupt your subsequent berserk rotation.