any particulate food must be measured on a case by case basic of course.
It really goes without saying. The mfg has no way of knowing what kind of nutrient export is going in in the consumers display (water changes, how much, or not?... skimmers working well or not, or not existing at all)... the ability of the consumer to finesse water flow to keep particulates in suspension for a maximum/optimal period of time, or not (sinking and polluting)... mechanical filtration trapping said food particles, then said food particles being rinsed/exported soon, or not (burdening water quality) etc.
I reckon the mfg guidelines for dosing phyto, feeding frozen or dry food, etc much like the warnings on a a cup of fast-food take-out coffee: it's obvious that the cup and contents are hot

And you are the best person to judge when it is cool enough to drink and how much to drink at a sip, etc
That all said... following the mfg rec's, we often see a new (uninformed/ignorant) aquarist with a young tank with a lower than average number or mass of filterfeeders than the "typical" display of the same filter-feeding reef inverts who likely will be overfeeding the phyto (and other foods). Badly sometimes.
We can't fault the mfg. They can only provide an educated estimate based on some average... and pray that we (consumers) use some common sense to finesse it for our individual systems.
And so... we should IMO treat dosing phyto like dosing Iodine, other food products, nutritive matter: add small and sparingly at first (I suggest half mfg dose) and then slowly push the envelope in time.
With Iodine eg., if you increase slowly over time to excess, you will eventially see an undesirable increase in brown diatom growth when you've crossed the threshold for iodine demand in your system. Indeed, in excess, something (diatoms in this case) will fluorish and exploit the boon. So we simply back down to the last known "safe" dose and ride it for a while until we think the need for iodine in the system has increased (biomass increase), when we can increase the dose slightly and see if the system sustains it (no pest or nuisance organisms flare).
In the case of excess phyto, we typically see a surge in sponges, fanworms, vermetid snails, and other filter- or mucus feeding organisms.
Look to such indicating organisms for guidance on if you are applying a useful or excessive amount of phyto.