Dosing skim mate actually works and so does stirring up the detritus in your sump to recirculate back to the display for late night coral feeding (I do this detritus feed about once a week myself). The idea is not about what was previously skimmed out or "over skimmed" but giving your system a nutrient spike the corals can use during lights out and the unused portion gets re-skimmed back out of the system by morning.
Again, the idea isn't to pollute your tank but as I mentioned to give a small nutrient spike after lights out that the corals can feed off of during the time they have their heaviest polyp extension.
It's called FREE FOOD.