I'd say the "academically correct" version uses implied volatility from black-Scholes, see here and then scaling it to your given time scale and taking the current price and +/- that percent of volatility, and you'd need to do the scaling for multiple time scales to get a cone. This would be how it would work for a forward looking chart based on what the options market is pricing in. You could also do similar scaling for historical volatility with the same multiple scaling to create a cone, although the black-Scholes version incorporates all news and other information that wouldn't have been reflected in prices used to calculated historical volatility.

But the main point is to 1) calculate volatility using one of the two methods 2) scale it to a time period 3) take current price and +/- the % volatility 4) repeat 2 and 3 for a few upcoming time periods to create the cone of possible upcoming prices.

I know this might be confusing so let me know if you have any questions :)