That depends upon what you mean by optimum.
1. You want tro drip fast enough to replace allo evaporated water, if that is how you are controlling the water level.
2. You don't want to drip so fast that the pH rises substantially, either locally in the sump, or in the whole tank. If the pH rises too much, you'll get precipitation of CaCO3 all over the tank.
So, the short answer is "add it just as fast as water evaporates, and no faster". You can add it somewhat faster, of course, but the slower, the less likely you are to have a problem.
