If you drop down to the small size barrel gravel vac, like the 1" diameter size, it will really slow down the rate of flow. With my large barrel, I get to half of the gravel at the point where my filter intake sputters and stops because the water is too low. (So that's like 55-60%!) In the smallest barrel, I can usually cover all of my gravel before 30%.
The large barrel really does a great job of suction though, pulling detrius out from under things too and giving a great thorough cleaning. But if most of the problem is the routine stuff that lies on the top, you should be able to pick that up with the small barrel and go through a lot less water.
To be honest - you may all disagree with me here - but I just do not buy into this 'don't vac all your tank's gravel at once' idea, if you don't have real plants, and probably never will for a tank my size. Why allow nitrates to build, and an excess of dissolved solids to form? I also think it's a little bit much to think you can gravel vac the bacteria off of the rocks. I have never had any minicycles in all of my thorough, 100% gravel vacs. I used to gravel vac it every DAY when trying to rid my tank of disease - the bacteria never suffered. One moderator one time told me... "you're a stronger woman than I if you can suck the bacteria off the surface." I believe her. :P
It's much more of a problem of keeping my NitrAtes down, and I'll clean the whole thing as thoroughly as I can, to try and do that :/.
(And big fish are the culprits. Schooling fish add a small amount, but I can live with that. Dwarf Gourami
pooh and sometimes even the Oto remains, drive me nuts to see collecting everywhere.)