Maths wiz please, probabilty formula how too

Ref: http://www.optionstradingsignals.com/understanding-implied-volatility-when-trading-options-part-1/

1 Std Dev = 68% Probability
2 Std Dev = 95% Probability
3 Std Dev = 99% Probability

This formula is fine for 1 Std Dev

1 Std Dev = Price x Volatility x SQRT( Calendar Days/ 365.25)

68% = Price x Volatility x SQRT( Calendar Days/ 365.25)

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Question1: Whats the above formula for 2 Std Dev ?

95% = ???

Question2: Whats the above formula for any probability percentage ?

75% = ???

NOTE: I assume you dont change volatility to fit, as thats a fixed variable.

Any ideas???

Thanks

 

2 standard deviations is simply standard deviation * 2.

 

75% is 1.152 sigma
from: <url>http://en.wikipedia.org/wiki/68-95-99.7_rule</url>

 

Thanks.. got it..

But I cant seam to calculate the 1.152.. hows that worked out ?

Please make as simple example

 

look in the spreadsheet.
The numbers are for the ERF function http://en.wikipedia.org/wiki/Error_function