Budget / Goal planning worksheet

Not sure if I am posting this in the right area or not but I am hoping I can get pointed in the right direction. I am using excel 2007 and want a spreadsheet that I can use for goal budgeting. My fiancé and I are planning our wedding and we would like a spreadsheet that would show us how much we need to save per month without redoing the math all the time.

Something where I can enter in our savings balance and it will figure the per month needs based on a set date. Our budget is $15,000 and we are hoping to have the wedding spring of 2013 or fall of 2013. It is easy enough right now to say 15,000 divided by 24 months means we need to save $625 a month. But I would like that per month number to self adjust as our savings increases or we get a large savings bump. Also we may not be able to do the $625 for a month or two so that will obviously affect the per month amount.

Hope that makes sense and I hope this is in the right place.

Thanks.

Try this.Wedding Budget.xlsx

Sorry just seen i made as error the formula in D2 needs to be corrected it should be E2 instead of C2. Should be as follows------> =IF(ISERROR(($G$2-E2)/B2),0,($G$2-E2)/B2)