By the
official jutsu doc, Shadow Clone can be used for training if it is at or above level 30, at which point our clones can last for 'a few hours'. Each training block of 'a few hours' with which we have a Shadow Clone training gives us +10% XP for that day. So if we gain 4 XP that day, then having one Shadow Clone training block in that day bumps us up to 4.4 XP.
With Wakahisa water, we have the theoretical ability to expend very large amounts of chakra to summon as many clones as we want. If we could summon 10 clones that last for 'a few hours' and have each clone spend 'a few hours' training before popping then we would gain +100% XP for that day, turning a 4 XP day into an 8 XP day. The catch is that popping clones forces us to make a Resolve check, and the more training blocks we have up that day, the harder the check is. Thus even with the chakra limit out of the way we are still limited by our Resolve.
(As an aside, if we bring Shadow Clone up to level 40 then we can have each Shadow Clone train for 3 'a few hours' training blocks in one day, instead of 1. This saves on chakra but does not help the Resolve check as one Shadow Clone training for three blocks is the same as three Shadow Clones training for one block each.)
So, the process of FOOM is the following:
- Train shadow clone at maximum capacity
- Dump all our XP into Resolve
- Repeat
This is a self-reinforcing feedback loop, as the higher our Resolve the more shadow clones we can field, the more shadow clones we can field the faster we gain XP, the faster we gain XP the faster we gain Resolve.
The main limiter on this feedback loop is that we need to maintain our skill columns. Right now our top skill is in the 40's, which means the highest we can get Resolve is 59 before we have to shift gears. Once we hit Resolve 59, the goal becomes to get something else into the 50's so that Resolve can hit 60, and then we return to leveling Resolve up to 69, and then repeat. It's in these segments that FOOM starts to really benefit us, and we've crunched the numbers to figure out exactly how much.
The overall plan for FOOM takes five years in-universe, and gets us enough stats to make S-rank (an example stat arrangement is Resolve 100, Deceit 90, Alertness 80, Sealing/Technique Hacking 70, MedKnow/Presence 60, etc.) However, we turn a profit in terms of 'XP spent on things other than Resolve' at the first diversion to build up our columns, as by that point we should be able to handle 15 Shadow Clone training blocks per day. You can find a spreadsheet (created by Noumero, profitability analysis added by me) detailing the progression
here.
Hope this helps!