well there may be a spot in between 30 and 100% that will get you going
an easy test to do would be to put your pump at full speed, and start some stress test, then reduce the speed little by little. The temperature of your GPU or CPU should go up just a little, but there will be a point where the temp will suddenly shoot up if you reduce too much. Just above that speed will be your lowest possible pump speed under load.
Then just add a bit of flow/pump speed to get some safety margin and you should have a fully working loop with very minimal pump noise, without even needing to vary the flow with load.
Depending on your waterblocks, a typical flow to cover all kinds of loads could be in the 80-120 l/h range.
you'll also see by looking at the temperatures that between that middle ground speed and full speed, there's almost no temperature difference. So it's not exactly worth it to run a pump at full speed.