Working with summed totals

#1
Hi, first post here - hopefully someone know this. Didn't know about this forum, but will definately spend some more time here :).

Anyways:
I'm making some estimates for 12 years using a regression model. The model gives me monthly predictions, with both confidence and prediction intervals.

Now I wan't to accumulate these estimates into a yearly estimate. To estimate the point prediction (I think thats the right word?) for the year I just sum the months. The difficulties occur when I wan't to measure the uncertainty by making confidence and prediction intervals.

I have different standard deviations for each month, so I think I'm able to use the stds to use in both the calculation of the prediction and the confidence interval - but how many degrees of freedom should I use when calculating the t-value?

The intervals calculated for each months were made based on ca 5 years, so I had about 60 observations. Therefore I used ~60 degrees of freedom for these calculations. Should I use the same number when calculating the t-value for the yearly intervals?

Don't know if this is an easy question, but hopefully someone knows the answer.
 
#2
Update:

Seems like I was wrong to use ~60 degrees of freedom when I have a total of 60 observations from 12 months (5 years). It seems to me the right number of degrees of freedom is n-1-groups = 60 - 1 - 12 = 47. I've verified this with calculations in minitab.

But still I'm not sure if I lose additional degrees of freedom when accumulating my monthly estimates (where I used a t-value with 47 degrees of freedom to make prediction intervals) into yearly estimates.

I've worked on this problem the whole week, and it's the only piece missing before I'm satisfied with my report (regression model) - so it would be very nice if someone could help me as soon as possible (close to deadline).