Enter the historical proportion for G Chart

Stat > Control Charts > Rare Event Charts > G > G Chart Options > Parameters

Enter the historical value for the proportion that Minitab uses to calculate the center line and control limits. If you do not specify a parameter, Minitab estimates the proportion from the data. You can enter a single proportion for the entire chart, or you can enter a proportion for each stage.

The proportion you specify is the event probability, which is the chance of an event occurring on any given day.