Suggestions of "10 times turnoever" and similar are simple rules of thumb. These are great for the guy who plans to buy a filter, plug it in and not worry abotu details...
If you wish to have a custom designed filtration system then totally and completely disregard such rules of thumb.
Consider each form of filtration: Mechanical, Biological & Chemical...
Mechanical - there should be enough water movement to move the physical waste into the intakes. There is no X gph or X turnover that acheives this. It will depend on the size of your fish (thus the size of their physical waste), the stype of intakes your filters use (removing water only off the surface will naturally require much more current to lift the waste than intakes that remove water from the bottom of the tank), even decor plays a major role here.
Also the placement of your water movers is a major factor. Strategically placed powerheads can be much more effective at a lower flow rate than returns that just hang on the top rim at a higher flow rate.
Biological - I hope not to create to deep of a conversationon this one, but in my experience bio media is nothing more than a manufacturers way of taking yoru money. Bacteria grows on anything/everything and in the vast majority of our tanks there is planty of surface area without bio media. The rare situation of a heavily stocked tank with no decor/substrate will be the exception to this idea, but if you have a typical amount of substrate and decor additional "surface area" through bio media isn't necessary.
Also, if you do use bio media (which most people do) then use it wisely. Thoroughly mechanically filter the water before you send it through your bio media. Allowing physical waste to collect in your bio media will allow your nitrates to unnecessarily increase... and allowing physical waste to collect in bio media will cause you to clean teh bio media more frequently and in the process of cleaning bio media it is inevitable that you will remove some bacteria which can obviously hold undesirable effects.
Chemical filtration - Is basically optional. If your tank/water smells it's a good idea to add some charcoal or if you are removing medications. It can also remove the oily film that commonly collects on the surface. But in very very few situations is it "necessary".
So consider the details. Filtration is not a game of numbers, it's a matter of effect. If you are removing the physical waste while keepign the ammonia & nitrite at zero... then you have great filtration regardless of your "turnover" or your "gph" compared to someone elses...
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