How do you calculate or interpret the results of standard deviation calculations

The mean value of a series of hemoglobin controls was found to be 15.2 g/dl and the standard deviation was calculated at 0.20. Acceptable control range for the laboratory is +/- 2 standard deviations. How do you find the allowable limits for the control? Would you please include how to do the calculations so that I will be able to do it as well. - Thank you

One SD is 0.2 . . . two SD's is 0.4

15.2 - 0.4 = 14.8
15.2 + 0.4 = 15.6

Limits are 14.8 d/dl and 15.6 g/dl

If I'd understood your Q correctly.