Filling under sink soap dispensers is a real pain in the ass.
It's great having a soap dispenser under your sink, but it's completely frustrating trying to fill the darn thing without it overflowing all over the place. I have two ideas to fix this problem.
First, the easiest solution is to have a pre-measured bottle that has just the right amount of soap, so instead of randomly pouring soap from a giant bottle, you could pour it from the smaller bottle and know for sure that it's not too much.
The second method is a simple valve that you can insert into the fill hole, that automatically shuts when its full. This is a bit like the valves inside gas pumps that prevent the pump from overfilling your car's gas tank.