Honestly I'm not entirely sure how the max-rank-up thing works, but from what I can tell, you can either focus on belly to boost just that way beyond the usual for a bloat run, the other two for an hourglass run, all three equally to just push from tier B to A normally, but after reaching either bloated, hourglass, or A tier elasticity, you can't go any further without popping unless you happen to have any cowbell item in your inventory, which can turn A tier into "Cowloon", which is the biggest tier I've found (albeit it has the drawback of requiring the lactation curse to be applied which typically means imminent popping). I kinda wish there was an S tier or something like that, but I don't think there is.