Parameters of gaussian distribution, which is generated using central limit theorem

In a software I am working on (sensor simulation), I needed to generate normally distributed noise for simulated sensor signals. I used the central limit theorem. I generated 20 random numbers and built an average out of them to approximate the gaussian distribution.

So I took the "measured" signal and generated 20 numbers from -noiseMax to +noiseMax and averaged them. I added the result to the signal to have noise.

Now, for my university, I have to describe this Gaussian distribution by its mean and variance. Ok, mean will be 0 but I have absolutely no idea how to convert noiseMax in my program into the variance. Googling haven't helped much.

I was not sure if SO is the right SE platform for this question. Sorry if it isn't.