What percentage of expenses should I factor in?

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What are some of the guidelines I should follow when trying to figure out if a deal is good or not?

Say for example i'm getting $2000/month gross from a 4 unit in my area (it's cheap around here)

That house would probably cost around $120,000, which makes about an $860 per month payment for mortgage with 0% down

What are some of the percentages I should be subtracting to get a rough estimate of the house?

All I know is 7-10% for a property management company and about 15% vacancy rate. For taxes, insurance, repairs, etc....NO IDEA

Can anybody give me a general guideline to follow?