Help in calculating risk on investment property

[Dana Leviel]

Ok guidance needed: I have purchased 3 investment properties (1 condo, a duplex, and a house) and helped with the purchase of many others. I alway take (rent-(Hoa,taxes,insurance,management fee, mortgage(if there is one)))/ initial investment to find my ROI. What I don't know how to include is unknown risk factors like repairs or vacancy, or ????? That I don't know how to account for. How are you guys accounting for and calculating these risks? Ps I never take into account the appreciation of the property, it's icing on the cake for me and not to be expected.