Forecast Revenue Data After Invoice Month

[tbloom]

Hello, and thank you in advance for your help. I'm attempting to actualize invoice data per month (easy part), and then re-forecast the target revenue for the remaining months (I hate this with a fire of a thousand suns). I'm currently able to forecast for the entire advertising campaign based upon a timeframe and revenue per month, but once I pull in 'actual' invoice data for the month, I can't seem to find an easy way to 're-forecast' for the remaining months based upon a change in a past month.

[TABLE="width: 500"]
<tbody>[TR]
[TD]Line Item (A)[/TD]
[TD]Rate (B)[/TD]
[TD]Targeting Impressions (C)[/TD]
[TD]Target Spend (D)[/TD]
[TD]Flight Date Start (E)[/TD]
[TD]Flight Date End (F)[/TD]
[TD]3/1/2015 (G)[/TD]
[TD]March Invoice (H)[/TD]
[TD]Revenue Goal Less March (I)[/TD]
[TD]4/1/2015 (J)[/TD]
[/TR]
[TR]
[TD]Animated[/TD]
[TD]$5[/TD]
[TD]1,000,000[/TD]
[TD]$10,000[/TD]
[TD]3/3/2015[/TD]
[TD]7/31/2015[/TD]
[TD]$1,933.33[/TD]
[TD]$2,000[/TD]
[TD]$8,000[/TD]
[TD]??????[/TD]
[/TR]
</tbody>[/TABLE]


Currently, I'm able to forecast by month using the following:
Column G
=(IF(AND($F2>=G$1,$E2<=DATE(YEAR(G$1),MONTH(G$1)+1,0)),MIN($F2,DATE(YEAR(G$1),MONTH(G$1)+1,1))-MAX($E2,G$1),0)/($F2-$E2))*$D2

My main question remains then, how do I adequately actualize the March Invoice (H) and give Column J (and any subsequent month) a new revenue goal? How can this stay consistent month to month without changing the formula (so when a new month is added, the projections are instantaneous with the new invoice data?). If you'd like to play around in it, I have a google sheet going: https://docs.google.com/spreadsheets/d/1kNVzcWHxKFXfK43MxsYgLim3JM3E0SGfr3-zD1tn2CY/edit#gid=0https://docs.google.com/spreadsheets/d/1kNVzcWHxKFXfK43MxsYgLim3JM3E0SGfr3-zD1tn2CY/edit#gid=0