As a bit of an aside there are a series of simple equations on the Mass-Spring-Damper model that will allow you to calculate the exact amount of damping-effect you need from your cornstarch ball to get the desired response from your system. Check out this Wikipedia link for some high level info and Google “Mass Spring Damper” for an endless supply of examples. Basically, any moving system can be boiled down to a simple MSD equation, and there is a whole sub-field of engineering called Control Systems that deals with this kind of thing.
It sounds like you already have the Springs in place on your device, and you have a known weight. Now you have to determine how much over-damping or under-damping you want the system to exhibit, and then fill/adjust your cornstarch ball to match the solution to that equation. Easier said than done probably… but at least you have the Math to back up your real-world system and you can analyse why and how things are happening the way the do.