forecast of savings balance

Hi,

I have a spreadsheet showing the balance of my savings account. See the graph of my balance as the blue line, the "projected" average progress as the yellow line and the goal line as the path I'd have to keep up with to reach my target amount by my target date.

My question is how can I better calculate my projected or forecast line so that it's more of an average rather than being so dependent on my monthly cycle of pay & expenditure.

Edit: I should add that the forecast in the graph is simply the straight line connecting the start point and the latest balance.

Also the height at which the lines hit the right hand edge shows me the final balance on my target date. So even if the forecast was calculated differently, it would still be helpful to see how high the forecast will be on the target date.

Thanks.

No suggestions? Maybe I'm doing it the best way already?