I watched many soma's games recently and I noticed that soma's muta army always builds up like 5,7,11...and not like 6,9,12... It makes sense because 5 mutas do one-shot marines and 7 mutas do one-shot SCVs. So the key number for muta army is 5 and 7. And 11 is obviously a number for full control group minus one overload.
That means macroing muta by spamming 5sm 6sm is not efficient. For example, when you have 3 mutas and have 3 larvae you should hatch 2 mutas and 1 drone instead of hatching 3 mutas. If you have 6 larvae you should make 4 mutas and 2 drones.
This is a great discovery.