Sounds fair. I have a private agreement in place with my DD'd father on $150pw until she is 18. We can cancel it at any time just by filling in a new form (both of us have to sign)
His assessment varys as he keeps changing jobs but last look he was supposed to pay around $106pw (so I'm in front)

For this bub I'll just be going straight through CSA and getting their assessment as he has a son already and I know his income.

If CSA do an assessment, you can ask for it to be reviewed and show your expenses etc. If 20% of his income covers all that, $60 doesn't sound right (I know someone who earns $75k and his amount is just over $500 a month for 1 child)