Will my spousal support decrease by 20% of every dollar taken in by my renting a room out in my home?

The best answer is it depends.  The first main factor is the agreement and subsequent orders that were entered in your case.  If you were able to negotiate a fixed amount of spousal support with an ancillary agreement that there would be no modifications, then you may be able to fight any motions to modify on the basis of the prior agreement and order.

If there are no limiting clauses in your order, then the court could potential modify your spousal support.  The court will look to a variety of factors to make this decision, including:  your education, your need for the spousal support, how long you were married, any abuse or violence during the marriage, any unique hardships, and any other "just and equitable" factors that the other spouse will raise.  After hearing all of the evidence regarding all of these factors, the court will make a decision.  The judge can keep the amount of support the same, increase the support, or reduce the amount of support.  The reduction, however, is not an automatic 20 percent.  The reduction or increase will be based on what the judge thinks is fair and just.

If your ex- does file a motion to modify, consider filing a counter-motion to increase support.  Renting rooms out of your home is risky (despite all of the self-help finance sites that suggest it).  You never know if the next tenant will be a crook or an abuser.  Your argument would essentially be that you have had to endanger your safety by offering boarding to strangers because you cannot support yourself otherwise.  If you had additional support, you would not have to take such extreme action to meet your needs. 

Your spousal support should not decrease by 20% of every dollar taken in by you by renting a room out in your home. Rather your spousal support should be based upon what your net monthly income is pre-renting versus post renting in comparison with the net monthly income of your former spouse.