Estimating cap rate?

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Hi all, a long time ago I read of a simple way to estimate the cap rate on a property, divide the gross rents in half then divide by the value or asking price on the property.

I'm curious, estimating expenses as half of gross rents, does that incorporate mortgage interest? Or should that be further deducted in order to come up with a reasonable estimate if you're taking out a mortgage on the property?

Also curious how well this estimate fares in your guys' experience in terms of accuracy?

Thanks