(a) Income from self-employment, whether positive or negative, includes benefits allocated to an individual from a business or undertaking in the form of a proprietorship, partnership, joint venture, close corporation, agency, or independent contractor, less ordinary and necessary expenses required to produce that income.
(b) In its discretion, the court may exclude from self-employment income amounts allowable under federal income tax law as depreciation, tax credits, or any other business expenses shown by the evidence to be inappropriate in making the determination of income available for the purpose of calculating child support.