A little late but an important thing to consider about revealing FOOM to Keiko is that we currently do not have the infrastructure to support even 3 initial FOOMS. The koi are currently providing 625 CR per day. The starting level of FOOM is 225 CR per day. I'm addition every member has taken the Yamaka training Jutsu that cost 20 CR per day. 60 for Noburi since he pays triple cost. That means for just Hazō and Akane we are spending 550 CR per day. And the cost of FOOM will just continue to increase. Until we have expanded our Koi farm significantly we fundamentally can't support more than 2 people FOOMing. So it is best to keep it secret from Keiko until we can expand our farm
The CR/day stat listed in the spreadsheet does not account for personal chakra regen. At initial 225 chakra per FOOM per day we don't even need the koi at all, since everyone FOOMing has enough CR to regen that amount within a day. Add in the Koi farm and we can afford to do that and still keep our FOOMing ninjas at above half chakra for any other expenditures of the day. Sure, eventually we will run up against higher FOOM costs per day, but that brings me to my second point:
Supposing that we can't keep maximum FOOM going for everyone at once. Okay, cool. That doesn't mean we exclude people from FOOM altogether until we have enough to perfectly accomodate them. It means we try for the optimal arrangement for everyone, whether that's 'everyone FOOMs at slightly-less-than-maximal-rate' or 'two of us FOOM at max speed and the third FOOMs more slowly'.
In my opinion, it is
far from convincing that if we can't afford max FOOM for Keiko we should cut her out of the loop. And that's assuming, of course, that we don't get enough chakra from ourselves + koi (which we do for the start) or that we can't purchase enough chakra from genin as FOOM costs grow (which we likely can). Not to mention, we have some control on how FOOM grows in when we seek SC 40. Taking a brief digression to level SC to 40 would, looking at the spreadsheet, essentially cut costs in half per person.
So overall I don't think this changes anything.
edit: also, think about how it'd look if this was the reason we gave Keiko. "So, we did the math, and there was only enough chakra for you to FOOM
half as fast as us. We decided that, even though we were in agreement on wanting to bring you in on this, it would be too awkward if there wasn't enough spare chakra for all three of us. Soooo, that's why we left you in the dark until now about our super awesome training technique to become demigods."