Yep, I also thought about that. Somewhere in the guide I say that I question the viability of last 1-3 lvls of CF. However, I still think CF should always be build (this is neither tested nor calculated, it is a guess based on my experience). There are several reasons for that:
1- You already mention it, after finishing the 80 arc you get 3 good GBs in your city.
2- Goods (this is very important). I do not think you can consistenly finish lvl 4 GE without these 3 GBs, and GE is a great source of FPs and diamonds. And even if you do, you need to produce HMA goods to trade for Arc and the other GBs, as well as donating to treasure whatever requirements your guild, events and daily challenges.
3- Diamonds. Definitely not the most important factor because you'll have get the diamond reward before building CF, but special events have also the chance of providing them.
4- CF at lvl 10 provides a high amount of coins. This means more than 4 non-feedback daily quests early game, when you have too many supplies (1 from gather coins, 3.125 from ubqs).
5- Let's assume you have enough goods. Then... I don't know, you might be right. I'm not interested in doing the math myself, but I'll check yours if you share it
.
Don't be demoralised because produce 2x24h is a small part of my quests now, if you forego CF, StM and LoA it will be much more significative. Also, take into account that my city has many TF and other special buildings, in the past I was doing more RQs than now (at least, RQs that don't include snipping
)