Computing error (excel or IRIS)

Hi all!

I'm very new at this, so probably this is a very easy question ;) :

I'm following the citizensky tutorial on DSLR photometry, using IRIS as my software package and the intermediate reduction spreadsheet. I stack around 10 images for each star and measure the magnitude on the final image.

But I don't know how to compute the measurement errors. Can I use the comp. stars fit value error and the check star error to compute the standard deviation of my measurement?

If not, how can I use IRIS to calculate it? I've read that using the "stat" command gives you the sigma value of the image, but I don't know what to do with that.

Thanks!