Forecasting Using Historical Seasonally Effected Non-linear Data

Forecasting using historical seasonally effected data

I organize my thoughts in numbers so it will be easier to understand.

[1] Say I have this data from January 2009 to December 2009:

1-4-7-10-13-14-15-16-18-20-22-25

As you can see, the growth is not linear.

[2] So then, I start with this data point for Janaury 2010.

26-a-b-c-d-f-g-h-i-j-k-l

[3] I now want to forecast that data point for the rest of 2009...using the growth in [1]. SO i guess that's an easy task right? Take the percentage change from Jan 08 and Feb 08...and apply that to the 26...and so on. (am i correct here at least).

[4] But lets say we have a GOAL. So, this become a forecast/plan. In December 2010, i want to grow double the amount of that in December 09. So 25 x 2 = 50. Now, the "known" data is like this:

26-a-b-c-d-f-g-h-i-j-k-50

So how do i do this now?