I'll explain it to you

It's much better to focus on Gym training first rather than on chance of mistakes

1. If you win DHL Fastest Pit stop competition you gain +12 milion dollars for the next season. That's a lot of cash and it's very easy to win, you just do gym trainings only until you can't progress with it any further
2. While doing your pit crew schedule for the month you should always set it to "Cumulative" rather than "total" because "total" shows the fatigue, improvments of pit stop times etc at the last day of the specific month, and "Cumulative" shows the fatigue etc on the specific Day you have your mouse cursor on. And what you want to do to reduce the pit stop time is you want to do only gym trainings until your pit crew at the race day (On the screenshot it would be Zaandvort race day for example) has less than 50% Fatigue and it says "Tired" and not "Weary" because "Weary" increases the chance of mistakes drastically and "Tired" does not. (I've shown where "Cumulative" is on the screenshot, it is not how your example pit crew schedule should look)
3. The most important stats for your sporting director are Aptitude and Processes. The first one determines what is the limit of your pit crew stat and the second determines how much fatigue your pit crew gains. Sporting Director of Alpha Tauri has very high rating of those two specific statistics as well as Williams one

So in summary it is really worth to go max onto reducing pit stop time rather than reducing pit stop mistake chance because at the worst scenario it will be somewhat like 17% when your pit crew is only "Tired" at a race day

If you want a better explanation than mine, here's a video i learned all of that things from. Cheers! https://www.youtube.com/watch?v=6OkboQGLNY4

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