Q: Can I include a step-child in the calculation, if my wife and I have sole custody and my wife is a full-time student?

A: The short answer is no for the stepchild, yes for the new bio child.

From Va. Code 20-108.2(C):

"Where there is an existing court or administrative order or written agreement relating to the child or children of a party to the proceeding, who are not the child or children who are the subject of the present proceeding, then there is a presumption that there shall be deducted from the gross income of the party subject to such order or written agreement, the amount that the party is actually paying for the support of a child or children pursuant to such order or agreement.

Where a party to the proceeding has a natural or adopted child or children in the party's household or primary physical custody, and the child or children are not the subject of the present proceeding, there is a presumption that there shall be deducted from the gross income of that party the amount as shown on the Schedule of Monthly Basic Child Support Obligations contained in subsection B that represents that party's support obligation based solely on that party's income as being the total income available for the natural or adopted child or children in the party's household or primary physical custody, who are not the subject of the present proceeding. Provided, however, that the existence of a party's financial responsibility for such a child or children shall not of itself constitute a material change in circumstances for modifying a previous order of child support in any modification proceeding. Any adjustment to gross income under this subsection shall not create or reduce a support obligation to an amount which seriously impairs the custodial parent's ability to maintain minimal adequate housing and provide other basic necessities for the child, as determined by the court."