Excel function to Sum

Alright, the title isn't too specific, but here's what I'm asking:

I am making an Excel template to calculate standard deviation, for a set of data to make homework faster. This is easy if the template is for a certain number of data points, but I wanted to make one where the user can enter from two up to, say twenty, numbers.

In one column will be the data, and in the next it will calculate deviation, using the formula "number-mean". Well if there is a number there then this works, but like I said there may not be a number since the template will work with a range of data points.

So how would I set it up, using an Excel function(s) that will only subtract the mean from the number if there is a number there? If I just use the formula I have now, =c9-d36 and there isn't a data point there the value will come out to the negative value of the mean, because it will take the mean away from 0, and 0 is not the same as no data point or "". This will make the calculated standard deviation incorrect.

Thanks for the help,

astro

PS - If anyone is feeling heplful I can send them what I have so far, and a copy of the finished product for future use. Of course, calculating the SD of a set of numbers is not something we use everyday.