I pretty much squirt it on while the platter is moving. The point is to make sure that there is enough to completely cover the surface of the record as you scrub.
I have been cleaning records for many years and, for what it's worth, here's my thoughts on record cleaning fluids: I have used most of them and have found that the ones that stay on the record well work best. The VPI fluid does not stay on the record well, and in my opinion does not penetrate into the grooves very well. The VPI brush is also not very good for using scrubbing force on dirty records. I have settled on the brushes that are now being sold at Music Direct as Mobile Fidelity cleaning brushes. I have found these to work the best with the 16.5. I am also using the Audio Intelligent cleaning fluids, which are sold here on AudiogoN. I get the best results by using the enzymatic formula followed by the super cleaner when cleaning a record, even a new one, for the first time. Importantly, the Audio Intelligent fluids stay on the records and penetrate well and I end up using less that half the amount that I did with the VPI fluid. The Audio Intelligent fluids are reasonably priced to begin with and, like I said, do the job better than anything else that I have tried.
I have not tried the Art du Son fluid, but having been in this hobby for over thirty years, I've become skeptical of products that make claims about superior performance and seem to cost way more than they should. It usually turns out that they just cost way more than they should. Remember, record cleaning fluids are made up of almost all water so any miracle ingredient should not cause an atronomical price tag.
By the way, I usually scrub for about a minute and vacuum for two revolutions on dirty records. When maintaining previously cleaned records I using scrub for no more than thirty seconds using only the super cleaner. I make sure that the fluid is distributed thouroughly and that the brush is wet before I start applying any pressure to the record surface.
I hope that this helps. Feel free to email me if you have any specific questions.