Optimal Timing for Phosphorus Fertilizer in Short Autoflower Cycles?

I don't think PK boosters do much of anything. Just feed it well all the time and no need for em.

You should be providing critical levels of P around the roots at all times. Significant portion of P intake revolves around "active transport". The plant has some sort of feedback system and takes in P and K in no small part through active transport as opposed to relying on diffusion alone (mass flow). This means that the best way to provide P or K is to keep a healthy level present around the roots, not necessarily in proportion to how it will be used in the plant. This is why overdosing P mostly leads to a lot of it going down the drain unused, and if there's limited runoff, building up around roots and causing a problem locking other shit out. Despite the belief that p and k need to be amped up in some esoteric complicated way during flower has been difficult to prove with real research. there are happy zones of concentration youw ant to stick to. if you are not providing that then you want to give more, but if already providing that more will only cause problems. Fairly sure this is why so many plants in mid to late flower look like burnt up pieces of shit, lol, yet it is somehow never a suspect when blame is assessed and some think it's actually a positive somehow to decimate their canopy with damage. Nevermind the supreme importance of CO2 intake and light absorption, bwahaha.

Sounds like they want you to use it right at the end of your flowering stretch.