Hello,
The 10X rule is a rule of thumb which which helps to guarantee that there is sufficient energy to move the fluid to most, or all regions of the tank. Flow rate should always be considered within the context of the distribution of that energy throughout the various zones of the tank (high, low, left, right, center and so forth). So having some arbitrary flow rate is meaningless if the distribution of that kinetic energy is uneven, because if the distribution and flow profile are incoherent then there will be areas that have too much energy while other areas suffer insufficient flow. In that case, both areas are susceptible to the risk of nutrient and CO2 deficiency.
By far, the most critical function of flow/distribution is the delivery of CO2 and Oxygen to the leaf. Terrestrial plants have the advantage in this regard because CO2 diffuses and transports easily in air. On the contrary, CO2 or Oxygen dissolved in water diffuses and transports at a rate of approximately 10,000 slower than in air. This is a major disadvantage for aquatic plants. Gaseous transport in plants is, by a huge margin, the most important mechanism to their survival and prosperity, so in water, the first trick is to increase the CO2 partial pressure, which is accomplished by pressure injection. Higher pressure helps to transport the gas but the diffusion and transport rates are still appealingly low.
There is a much less well known obstacle to the movement of the gas in fluids. Fluids moving across a solid object have a frictional component known as viscosity. We all know the term from motor oil commercials but it applies to any fluid. This friction slows the movement of the fluid to nearly zero at a short distance from the surface of the object. In our case, the leaves will have a layer of stagnant water surrounding it, referred to as the boundary layer. Boundary layers having near zero velocity also therefore have a high static pressure. This high pressure pushes against the rest of the water above it and is an impediment to the movement of nutrients and CO2 from the larger water area. So, CO2 and nutrients inside the boundary layer will slowly move across the plants exterior membrane but replenishment of the gasses and nutrients from the free stream into the Layer are impeded due to the high static pressure of the Layer. As fluids are put in motion their static pressure decreases and the Boundary Layer thickness becomes thinner up to a certain speed. Above that speed, the Boundary Layer again thickens up. So there is a small range fluid velocities near the leaf surface where increased flow reduces the thickness of the Boundary Layer and facilitates transport of nutrients/CO2 to the leaf. At velocities below this range the thickness and pressures are high, and above the range the thickness and pressures also increase.
Clever distribution schemes therefore allocate enough energy to all locations so that the fluid velocities are within the acceptable range in all the zones.
We normally advise that if all the leaves can be seen to be gently swaying, then this is an indication of good flow distribution. If some are moving like flags during a storm while others are static then this is an indication of poor distribution.
You might want to have a look at the video in the post
http://ukaps.org/forum/threads/inline-devices.2965/#post-32127 to get a better illustration of flow theory.
Cheers,