Luke Skywalker, Hero of Yavin: Double Attack, Twin Attack, Flurry Attack.

My dad started firing with Luke and on his first attack, which was doubled by twin attack, he misses both, but on his 3rd dice roll(2nd attack first half of twin attack) he rolls a 20. I am having a hard time finding how to play this out. Flurry says immediate and uses keyword "extra attack" but hes not done with his 2nd attack twin attack second roll. Here is how I ruled it but need clarification:
I went ahead and added an extra attack to the queue of attacks but made him resolve the other attack of twin attack (thankfully was not another crit complicating the matter). Now he was on his 3rd attack (5th dice roll) and since glossary says any extra attacks granted by abilities benefit from twin he got twin on this 3rd attack. So 6 attacks total now and on the 6th roll he rolls another 20 so I gave him another extra attack benefiting from twin attack once again for a total of 8 attacks and over 100pts of dmg dealth to poor vader. Is this how flurry attack and twin attack interact with each other?

Yep. That's it.

Pretty crazy when it works. Hero of Yavin has massive (unlimited, actually) damage potential. His low attack and hit points is his problem. He's the proverbial glass cannon.