I'm afraid it cannot be answered in abstract. it depends on the VA content and molecular weight of your EVA, on its MFI, on the amount, specific surface and surface treatment of the silica filler, on the compounding method you plan to use and on the property you wish to optimize. I think you cannot avoid experimenting. The order of adding the components may also be important. You can pre-mix the filler and the additive in dry blend form and add them together or you may melt mix the matrix and the additive and add the filer afterwards - the results will be different. Anyway I would suggest to try 1%, 3% and 10% of the filler weight.