Very happy to help.
As I understand it's the net profit figure but before tax is deducted, i.e. the taxable profit declared to HMRC.
If your pension and the rental profits form the totality of your income, it's a percentage of the overall income as I understand rather than one or the other.
This is lifted from their own guidance:
Unless a paying parent gets certain benefits, we work out child maintenance using their taxable gross annual income as the starting point.
By ‘income’, we mean earnings from employment, self-employment (profits from a business), occupational or personal pensions and certain benefits.
Gross annual income is the paying parent’s yearly income before Income Tax and National Insurance are taken off, but after occupational or personal pension scheme contributions are taken away.
So if you are still paying into a private pension, this would be deducted before your liability is worked out.
I hope that this clarifies the point for you?
Thank you for your enquiry today. I am happy to answer follow-up questions - please do get in touch with requests for extra information or further queries and I will do my best to help you.