Assuming you've checked through the literature and can't find anyone who has done this for your protein before, you'll just have to do by step-wise titration. It's impossible to predict with any reliability how a protein will behave during ammonium sulfate precipitation.
Start with a 30% ammonium sulfate cut. Centrifuge and assay pellet and supernatant for your protein (activity assay, gel, affinity column, etc). Assuming it's in the supernatant, add additional salt until 50% saturation and repeat. Then 70%, then 80%, etc. It's a bit laborious but if you do it correctly then you only have to do it once to find your answer.