Depends. What do you feel comfortable with. Let's illustrate...
Supposed one of your parents, a lifelong faithful member, pased away and left you $10,000. You know that they paid tithing on their increase throughout their life (in effect this money was originally tithed against and is a remainder) however, you have not tithed on this increase for you.
So you pay 10% or $1,000
Leaving you with $9,000
You have 3 children and decided to pass on some of this to them. You give each child $2,000 from their grandparents. Each child, also faithful members, pays 10% or $200 each. Leaving them with $1800.
Each of your 3 children have 4 children and they each decide to pass on a bit from the great grand-parents. They give each child $150 or a total of $600 per family. Each of those children, being young in turn pays tithing, or $15 for a total of $60 per family, or $180 total.
Your parents generosity of a $10,000 gift has resulted in $1780 of tithing or 17.8%, not counting the tithing that they originally paid.
Now some might consider that justification, they might be right. But I have also seen a wide variety of definitions as to what consititutes increase. Some say gross, some say net, others say that it is revenue-expenses that equates to income and thence increase.
The real answer is, it is between you and the Lord, and NO ONE ELSE.