There is no definitive answer. The Church is very nebulous about this - it's about our "increase". Some people pay on gross others pay on net. (Some say do you want gross blessings or net blessings. ;-) ) Some pay on Social Security income - others do not. Country-specific differences come into play as well. In Europe where the government pays for more things (healthcare, etc.) - do you tithe on that or not? 
 
It depends on the individual and how they interpret it. The Church only asks "Are you a full tithepayer? (leaving the definition to the individual." 
 
Here is the definition of Tithing from the Handbook: 
 
The First Presidency has written: “The simplest statement we know of is the statement of the Lord himself, namely, that the members of the Church should pay ‘one-tenth of all their interest annually,’ which is understood to mean income. No one is justified in making any other statement than this” (First Presidency letter, Mar. 19, 1970; see also D&C 119:4). 
 
Hope this helps.

Only the Holy Ghost can guide you on that, the Lord commands us to return 10% of our increase to him as a sign of out faithfulness and promises to bless us greatly for this.  
 
What is an increase though? Like you said is it anything you earn, is it just the amount left after taxes, or is it the amount left over after all the bills are paid? 
 
In each situation the word increase can logically and rationally fit. That is why the Holy Ghost is needed and much study and pondering of the scriptures required to understand personally what the Lord desires of you.  
 
For me an my house a full tithe is 10% of anything that enters our home. If I sell a book for $10 to a friend $1 goes to the lord. If I win a $50 gift card at work I add $5 to the tithing for that week.  
 
Even tax exempt income like my VA disability check is tithed.  
 
When I was on unemployment that was tithed as well. I tithed the proceeds of an insurance settlement from my motorcycle accident. I did not tithe the part that went directly to the hospital or that was used to fix my motorcycle since that was not an increase but simply what was needed to make me whole again and replace a loss. I struggled with that for weeks before receiving the payments, and much pray and scripture study and the confirmation of the Holy Ghost is what made me know that I was choosing the right.  
 
That is why the Church leadership can be so simple with there instruction on tithing each member has the gift of the Holy Ghost to help them personally make these decisions!