I'm not going to argue against any of the posts or the "common knowledge" about not dumping with low levels.....
but I have noticed something that puts a bit of a question mark in all of that idea of needing a full tank to pull the stuff out..
A long time ago at home I did some experimenting. This was done when the rig was still fairly new and at that point I had always waited till more than 3/4 full to dump.... I've dumped, rinsed, and repeated a few times trying to really get a clean tank. I really can't answer why because I don't really care if it's spic n span...it's a black tank.... except that I'm bugged by having level lights that don't work.... yes, it's a psychological issue I know.... Anyway through a clear elbow connector ... I've noticed that the water will run relatively clear through most of the dump, then lots of remaining stuff will come out as the flow slows to just the wash across the floor of the tank. Refill, and this repeats. I've done it several times in a row and even after several extremely clear dumps, still there will be a surprising amount of paper and stuff coming out at the end...not so much during the fast part of the dump but at the end, even after 3-4 clear rinse dumps or more.
So
I'm guessing what happens is that during the full part of the dump when the water is really rushing out it's probably mostly just a vortex pulling from the upper and central regions of the tank's space and the region of liquid with the solids settled towards the bottom are more or less bypassed until the water level slows enough to stop the vortex and just slowly wash across the tanks floor.... then and only then does it pick up the stuff from the tank floor.
Based on these observations and my guess as to what's happening, I personally don't figure it's the end of the world to do a "short" dump now and then....as long as whatever in there is still mostly liquid. I wouldn't do it if the tank level was nearly empty already,but half, or maybe 1/4 now and then maybe... I always try to wait till full or close, but sometimes that just doesn't work out....But in the rare times that I do feel like my dump was a bit early, I'm extra careful about adding lots of clear water and maybe some detergent right away to the freshly dumped tank to prime it....and when possible will try to do a rinse dump fairly soon after...