Again, download the free user's guides from the major formulators.
Each mix is dependent on several factors, so percentages are meaningless. Once you learn what each material does, you'll have an easier time figuring out what to use. There's also an experience factor involved.
Silica (cab-o-sil) is best used to control viscosity, so you usually don't need much. Fairing compounds are all light weight materials, that also are easy to sand. Each mix, regardless of application will usually have more than one material in it.
Fairing compound will be mostly the light weight materials (balloons, Q-cells, etc.) with enough silica to get it to do what you want. So, if you need to make fillets, you'll use enough silica to make a peanut butter consistency, but on flat surfaces, you'll only use as much as you need to keep it from running or sagging, which may be like a runny ketchup or heavy cream like consistency. Each batch will be custom mixed for the job at hand.
Now, you can premix fillers, which is what I do, each having different physical qualities. I store these in gallon jars and dump out what I need, depending on the job. I'll add silica to get the viscosity I want. A heady feature of this technique is consistent batches, but you have to justify mixing up large quantities of filler materials for this approach.
