The requirement relates to the sponsor's income, not their outgoing expenses. This means that regular income received from a rental property, even if that property is mortgaged, is still income and therefore can be used as F4.30.1iii states - either on its own, or in combination with other forms of income such as sustained self-employment.
The immigration instructions refer to gross income rather than net income, and this is how it is interpreted, so no form of calculation is required regardless of any mortgage that may be on the property.
I hope this information helps, please feel free to contact us should you require any further assistance.
Kind regards,
Louise McConachy
Customer Service Officer
Immigration Contact Centre