The amounts in Section 4062 shall be considered additional support for the children and shall be computed in accordance with the following:
(a) If there needs to be an apportionment of expenses pursuant to Section 4062, the expenses shall be divided one-half to each parent, unless either parent requests a different apportionment pursuant to subdivision (b) and presents documentation which demonstrates that a different apportionment would be more appropriate.
(b) If requested by either parent, and the court determines it is appropriate to apportion expenses under Section 4062 other than one-half to each parent, the apportionment shall be as
(1) The basic child support obligation shall first be computed using the formula set forth in subdivision (a) of Section 4055, as adjusted for any appropriate rebuttal factors in subdivision (b) of Section 4057.
(2) Any additional child support required for expenses pursuant to Section 4062 shall thereafter be ordered to be paid by the parents in proportion to their net disposable incomes as adjusted pursuant to subdivisions (c) and (d).
(c) In cases where spousal support is or has been ordered to be paid by one parent to the other, for purposes of allocating additional expenses pursuant to Section 4062, the gross income of the parent paying spousal support shall be decreased by the amount of the spousal support paid and the gross income of the parent receiving the spousal support shall be increased by the
amount of the spousal support received for as long as the spousal support order is in effect and is paid.
(d) For purposes of computing the adjusted net disposable income of the parent paying child support for allocating any additional expenses pursuant to Section 4062, the net disposable income of the parent paying child support shall be reduced by the amount of any basic child support ordered to be paid under subdivision (a) of Section 4055. However, the net disposable income of the parent receiving child support shall not be increased by any amount of child support received.