Hi,
Unfortunately flow rate reduction is a fact of life. To work your way around this you will simply need to use a stronger filter to compensate. There is no way to correlate a "normalized" water quantity to filter media. The reasons include the fact that not all media are created equally, so that some media have more surface area than others and can support higher bacteria populations, thereby doing more nitrification than a lesser media. Another factor is that filtration requirement is also highly dependent of organic waste production within the tank therefore, two tanks of exactly the same water content can have vastly different loads if one is heavily stocked with plants and animals while the other is lightly stocked.
Any liter(media) per liter(H20) calculation therefore would be completely irrelevant.
If you follow the 10X turnover rule of thumb you can then fill your filter with high quality media and forget about trying to balance media volume as a function of tank volume. Ergo, multiply your tank volume by 10 and this should be the turnover rating of your filter.
If the resultant filter size is beyond your means then the flow rating can be supplemented by adding lower cost powerheads.
Cheers,