One meaning of the standard deviation of a gamble is that it is about as risky as a fair coin-flip for the standard deviation. A HU SNG is very close to a fair coin-flip for the entry fee (which is a little less than the buy-in, differing by the rake). The standard deviation does not change much if you win 45% or 60% instead of 50%.
If you want to calculate the standard deviation from past data, one method is to calculate the following:
A = average result
B = average square of result
Variance = B - A^2
Standard Deviation = sqrt(Variance)
I have done that calculation for SNGs with 6, 9, and 180 players in past threads.
There is no time you need to use the standard deviation at the table. However, it shows up when you are analyzing your results. For example, a rough 95% confidence interval for your ROI after n tournaments is about your observed ROI +- 2 standard deviations/sqrt(n). If you are playing HU SNGs with a standard deviation of 0.95 buy-ins, and you have played 100 tournaments, that would be your observed ROI +- 190%/sqrt(100) = observed ROI +- 19%. That tells you about how well you can pin down your ROI after playing 100 tournaments.