What is the deductible amount?

The “Deductible Amount” field that you will see for Income/Other Income items when the Tax Treatment is set to “Wages” or “Self-Employment” is the amount from the paycheck that won't be subject to income tax. These are pre-tax expenses and amounts deducted from a paycheck before income tax. The most common examples include pre-tax 401k contributions, other pre-tax retirement account contributions, health insurance premiums, dental premiums, etc.
 

FICA for Self-Employment Income

Those earning wages from an employer (i.e., not self-employed) have only half of their total FICA  (Medicare and Social Security tax) withheld from their paycheck. The other half is paid directly by the employer. In contrast, those with self-employment income have to pay all of their own FICA tax on that income. Because of this difference, one half of the FICA tax paid by self-employed individuals is deductible against ordinary income for the purposes of ordinary Federal income tax. Income Lab automatically handles this deduction, so you should not include this amount in the “Deductible Amount” field.