Formula for the model used in this calculation is 0.002 * difficulty ^ 0.51. It is possible to change this numbers if necessarily.
In true TradingView spirit, the author of this script has published it open-source, so traders can understand and verify it. Cheers to the author! You may use it for free, but reuse of this code in a publication is governed by House Rules. You can favorite it to use it on a chart.