How do they calculate how much you pay for National Health?

I'm wondering how the British government calculates how much each person pays for National Health - I understand people pay different amounts based on their income? I may be totally wrong!
Here's my situation:
I'm married to a British Citizen and will be heading over to England to live in January. I have a son from a previous marriage, and I need to figure out how much child support his father should be paying. In Florida, they calculate child support based on how much the mother and father make, and add in child care and health insurance/care costs. I'm trying to figure out how much to calculate for my son's healthcare in England. Will my hubby's payment to National Health go up when we move over there? How does it all work?? Thanks in advance for any help and advice...

There is a difference for between National Insurance and how the NHS is funded.  NI contributions under the PAYE are totally calculated by how much someone makes, there is no longer a difference in NI Contributions based on if you were married or not (and this only applied to women).  You can read all about how the contributions are calculated here: http://www.hmrc.gov.uk/nic/work/ni-uk.htm

Now all the people who earn wages and make NI contributions in the UK fund a pool of money that in part pays for the NHS.  These monies are then given to local governments to reimburse heath care professionals for their services and pay for prescription drugs.

So your presence (and your son's) has no effect on your Husband's NI contributions.  Now if your husband received private medical coverage from his employer, then depending on the program, he might have to pay some additional funds in order to cover you and your son, if you so desired.

The day you and your son arrive in the UK to settle, you will be entitled to not pay for any NHS services (if you are just visiting, you are not covered).  Basically you are moving to a modern country that feels that health care is a basic human need.  I know it seems strange to us Americans, but it does actually happen.

It's a popular myth that NI contributions fund the NHS. The NHS is funded out of general taxation (VAT, Income Tax, Corporation Tax etc). NI contributions pay for pensions, maternity, sickness & unemployment benefits.

NHS is intended to be 'free at point of use' so once your son is eligible you won't need to claim help from your ex.

That is what I like about the UK.
I personally couldn't consider becoming a US Citizen, I love the countryside , The lifestyle, Scenery, so muct to see and do.
I could not however could live with the Insecurity of emloyment and expensive medical insurance, to be scared of being seriously ill and becoming bankrupt by it, is no way to live IMHO.