hi, my ex and i have a daughter together and i have gone through child support and that is being handled through them. he has an attorney and they

created a joint parenting agreement, i am ok with everything in the agreement execept for the cash payout portion, i feel we can delete that or just word it saying 20% of his salary instead of listing amounts since it says until age 18. the reason is i dont want to lock myself into a set amount for 18 years of her life, i know child support reviews every couple of years and i can reopen if i feel he is making more or circumstances changed, his attorney told me that the joint parenting agreement can be modified and that we dont need to change the amounts that since i am going through child support in illinois this wont change anything, i dont know if i can trust this, is he right?