Predicted Probability

Someone created a spreadsheet to graph predicted provability values on a Logistic Regression S Graph. There are four values used.

a = Assumed Maximum
b = Assumed Minimum
c = Raw Data St. Dev (Field Data)
d = Model Output (Research Data)

the formula used if the following

NORMSDIST (a-d) / (((c^2)+(b^2))^0.5)

Wondering why the data is being manipulated by the formula & what is accomplishing

Any help will be greatly appreciated. I can provide more detail if required

Thanks for replying to my post... :)... I'll try to simplify/clean-up the spreadsheet I have & post it. In the mean time, the attached equation might be a bit easier to understand?