First, I've used this method many times in quite a few ponds now.
It's not linear as for the amount of Alum used vs what it sinks. Either you have enough for the particles to bind with or you don't.
Think of it like a course spider web, or 1" casting net. As it falls through the water, most particulate just goes through the net whereas a smaller mesh it builds on making the holes smaller gathering more as it goes down. It has to flocculate to work.
Alum is a positive charge by +1 because Alum is +3, sulfur -oxygen is -2, but sulfur and oxygen are non metallic so the positive charge is transferred to those elements.
Clay is a negative charge. and is attracted to the alum. Not enough Alum and the clay particles crash into each other with insufficient force to make contact and "stick".
In most cases, Type S Hydrated lime should be used as a pH buffering agent at half-rate of Alum. Great part is this adds to the positive charge and attracts even more clay in suspension to sink.
The ions only have so much power to attract and depending on the source of suspended clay, you'll be doing it twice if you don't use enough the first time.
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