Sangamon, like Cook, require a special affidavit for child support. Since we're dealing with a fluctuating variable (overtime), one party is going to up-play it, the other will down-play it. That's where a good lawyer comes in and states that past performance is no guarantee of future results (in regards to overtime) and should be disregarded.
When I have a divorce and one party works over-time, I calculate based on the W-2 from last year, as well as the paychecks earned in this year. The court can then hear the difference in income. So, the overtime would be calculated in that figure as well. Any Child Support Order would require that the percentage to be taken from your check (depending on how many children) will be taken from your regular check as well as any overtime check when it's issued.