How to calculate standard deviation and use three-sigma rule for couple variables?

Question

Generally, I have the following data:

CampaingOne 49%
CampaingTwo 41%

I need to use the three-sigma rule and check if the second value is in the interval [- 2 sigma;+ 2 sigma].

As I have read, the sigma value according this rule is as follows:

1 sigma - 66.7% 
2 sigma - 95%
3 sigma - 99%

So, what I am doing is to check how many percent is the score of CampaingTwo from the score of CampaingOne using the following calculator.

For example:

41 from 49 is 83.6734693877551%

And now because the result is little then "2 sigma" I am concluding that it's not in the interval.

Anyway, I am almost sure that what I am doing is far away from the truth and need some help in this.

Could anyone give me a simple example of what calculations I need to perform?

Answer 1

I will consolidate my comments here.

Since the standard deviation ($\sigma$) is calculated from the mean of the data, usually the three sigma rule is also based on the mean of the data.

From the data given, the mean is $\dfrac{.49+.41}{2}=.45$.

The standard deviation would be $\sigma=\sqrt{\dfrac{(.49-.45)^2+(.41-.45)^2}{2}}=.04$.

If there is different mean, there must be other data, which might mean a different $\sigma$. Otherwise, you can compute the distance between the two samples in terms of their standard deviation using their difference ($.49-.41$) the value if $\sigma$ computed above.