I usually aim for 10x from filters and power heads.
I am using the manufacturer's labeling, which we all know is over rated.
Aim the combined water movement in such a way that it is not cancelling each other out.
Actual filtration should be slower, with the water well spread out over the filter media. This benefits all the filtration we are doing.
Mechanical: If the water flows too fast it can knock the debris off the sponges and floss. Best to have a wide surface area and a deep thickness of mechanical media.
Chemical: If the water dwells among the chemical media longer it gets the benefit of either removal or addition (whichever you are doing).
Bio: Too fast water flow damages the bio film the beneficial organisms live in. They need a good flow to bring them the oxygen, ammonia and other things they need, but not too fast a flow.
Water circulation that emphasizes top to bottom circulation is most beneficial for gases that the water exchanges at the surface.
Side-to-side water movement is more natural for the fish.
Water movement across the bottom should bring the debris closer to the filter intakes (usually filter intakes are near the bottom).
I like this answer best. As my water flow is created by my filter AND I have seen the effects on 'debris in the water' by more powerfull filtration in my tank I will keep the status quo for the time being and look in to a purely water movement feature in the future.