Tropica did one of these a few years ago and found the optimum flow
for plant roots was about 0.49liters/day/m^2
Not much. Cables cause much more flow than this.

I've used cables from Dupla, Sandpoint, reptile heating pads, built my
own cables(7 over the years) and I feel you don't get anything for the
trouble or the money.

I've have done a number of flow rates through gravel over about a
decade.

No flow, osmosis is the most popular and easiest and the best IMO.
Low flow, cables are good but I saw only some added stability in the
start up, the theory behind it and the observations themselves are
weak at best. But it does seem to hurt either.
High flow, RFUG. work great also. I used these for about 15 years and
really like the results in the start up and the long term.

The best substrate I have used and still use is Flourite or onyx sand
with some ground peat and rich vacuumed mulm from an established tank.
That works the best IMO. Never breaks, nothing to plug in , grows
plants the best etc.

Unless you have a closely controlled temperature in your home, the
cables will be shutting on/off etc and that will greatly effect the
flow rates.
Many folks cannot use them in the summer due to high temperatures etc.

Plant roots transport a good deal of water and nutrients in/out of the
gravel also.
Regards,
Tom Barr