Once you have all of the necessary documentation, you'll need to calculate your monthly income by taking your average annual income and dividing it by 12. This number will be used as the basis for determining how much you can borrow. Depending on the loan type, you may need to provide net income instead of gross. The amount subject to self-employment tax is 92.35% of your net earnings. You calculate net earnings by deducting ordinary and necessary expenses from the gross income you received.