Formula for budget trending

I have a very simple spreadsheet that shows budgeted amount for a month, billed amount, and an over/under amount monthly. I have these summed in the end column for total budget, total billed amount and where this ends up over/under budget. I'm trying to figure out a formula that calculates based on the total amount billed, if the spend continues as budgeted over the remaining months, will we end up over or under budget? I've attached a sheet showing a sample. The cells highlighted in yellow are what I'd like to populate. Again, based on actuals spent, it we continue to spend the budgeted amount for the remaining months, are we trending over or under budget. Thank you.

That doesn't quite work out. Example...I filled out the remaining months in the "billed amount" with $500 per month as budgeted, expecting then to show that I would end up $200 over budget in I4...but the average function showed me at -33. Thanks.