I think we are talking at cross purposes here. The short answer, in my opinion, is that you can decrease the numbers to 85% and the variability will automatically decrease to 85% without you doing anything, or you can use the untouched numbers and decrease the mean and SD at the end.

If this doesn't make sense in your context, then perhaps you could explain the actual project more fully.

Let me explain the project more fully.

The model I made is a Monte Carlo Simulation (10,000 times) and is used to show two teams playing each other and the probability of either one winning. Winner is determined by who has a higher final score for each simulation

Here’s what I do: I collect each teams AVG Runs Scored (Offense), AVG Runs Against (Defense), and the Standard Deviation of these numbers as well. I pull this information from finding every single game each team has played thus far into the season.

Now, I will find the average of a teams AVG Runs Scored and the other teams AVG Runs Against. This gives me an understanding of Offense vs Defense and now I have a consolidated value. I also do this for the STD DEV Runs Scored and STD DEV Runs Against, I find an average of the two.

At this point I have an AVG Predicted Runs and STD DEV Predicted Runs for each time.

My next step, and this is where I get confused, is I now look into the starting pitcher each team is facing. I will find the league average ERA, and divide the starting pitchers stats by league average ERA. If the team is facing a pitcher with an above average ERA, that pitcher’s value relative to league average will be above 1, let’s use an example and say 1.15

I will then multiply the AVG Predicted Runs for team that’s facing this pitcher by 1.15. I expect their Predicted Runs to go up because the pitcher they are facing isn’t very good. My question is, do I multiply the STD DEV Predicted Runs for this team by 1.15 too? Or not? And why? I just don’t see why I would but I’d like to confirm that so I can be more accurate.

Does this help clarify a little more? I can also send you the Excel file!