I had actually made up my mind to quit this game if I didn't manage to summon any New Event Hero in my first 14 summons for this event.
For the first 14 summons in my last two events, I did not get a single New Event Hero. Someone datamined and found out that the percentage to get a Rate Up hero increases if you don't get one after each summon.
With my subsequent Summoning videos, I will try to investigate and find out at which point will the number reach 100% and it is at that point which you will have a chance of summoning either one of the Rate Up heroes.
This reminds me of the UFO machines in Taiwan where after a certain number of tries, the grip on the UFO will become tight enough for you to definitely be able to get a soft toy. However, the only difference is the UFO machines do allow for some skill to be involved in getting the soft toys as well even when the grip is not tight yet.
I feel that the percentages for summoning the New Event Hero and the other Rate Up hero should not be linked together because honestly speaking, who wants another Kahuna. It would be more justified to have them seperate so people don't feel so demoralised when they summon for 14 times and find that they are being given 2 Kahunas. They would definitely hate the game less if after 14 summons, they got 2 Kahunas and 2 Zhuge Liangs instead.
Anyway, back to the statistics, I summoned a Zhuge Liang in my third summon. My next Zhuge Liang summon was on my 13th summon. This goes to show it at least takes 10 summons before you hit 100% chance of summoning a Rate Up hero. No Kahunas were summoned throughout.