His "earning capacity", quite simply, is the amount he could be expected to earn based upon his skills and experience.
You say that based on his profession he could be expected to earn at least 30K / year.
So your arguement to the CSA / court is that he has an earning capacity of at least 30k, and you want your award to be based on that figure....not on his deliberately reduced declared earnings.
You will need to back your arguement up with:
- some salary survey data
- as much info as you can on his current contract
- the current demand for his skills: get a recruitment agent/contract agency to provide a short letter stating that the market is good at the moment.
- go to a job website (or newspaper) for his line of work and print off many pages of open positions
- gather proof of his actual achieved earnings in last 5 years