To work backwards to find realized income needed from a given mortgage payment amount, you use proportions.
For example:
Say you are paying $1250 as monthly rent.
So, if you spend 25% of your monthly income on rent, you need a monthly income of $5000. If you can spend 30% in rent, then you will need a monthly income of $4166.66.
This way proportion is used to find the required answer.
To work backwards to find realized income needed from a
given mortgage payment amount, you use proportions. I am hoping
that this answer has satisfied your query and it will be able to help you in
your endeavor, and if you would like, feel free to ask another question.