I find getting the right amount of "springiness" tough when I experimented with springs under my TT (very heavy plinth made of multiple layers of Baltic birch plywood). If the spring is too soft it just compresses under the weight and no longer behaves like a spring as it should. If the spring is harder then it produces too much bounce. The trick was to tightly insert just the right amount of foam rubber inside a hard spring to dampen it.
Another very real struggle was finding the right locations to place the springs since the TT has very uneven weight distribution. A TT must be level for the cartridge to track properly. It took lots of experimentation to level the turntable.