@magic
Thanks for the detailed explanation of the resource integral. IIUC, what you are optimizing is total amount of resources from one planet over time, assuming that you hold at X% to fill freighters until you have enough pop to fill to 100%. Is that correct?

My scenario was that you don't have good breeders anymore, so pop goes off to marginal worlds that only contribute to long term growth, while you also need some resources to research/build the stuff you need to reach said long term. In your advise, you say hold at 33%, which is of course the way to fill those marginal worlds asap.

My reasoning for a hold around 42% (I agree there is no magic at 42% like there is at 25/33) would be that you want the planet to produce a good number of colonists while also having good resources/mines.

33% is still optimal, of course, but in absolute terms you only lose 5% of max absolute pop growth, while gaining around 25% production.

Of course, the optimal value for jointly maximizing production and pop growth depends on the relative weight of either. Moving further to 50% you gain another 19% over the resources at 33%, but at a cost of 15% of colonist production. This is presumably optimal as you point out for the capacity of this planet, but in most cases I would want to keep one breeder around to top up the nearby good worlds anyway, so considering the resource integral in the context of only a single planet is not so useful.