Over the past year or so a funny term seemed to be coined in the 40k community..the so called 'math hammer'. It is where you calculate on average what should happen. Theory crafting out lists based on using some math hammer skills is great for conceptual ideas. I use it all the time to figure out what should happen.
However, may people try to use it in a tourney setting to try and figure out exactly how and what you should do. This is just wrong. You can think all you want all day but in the end the two important things that you should have instead is your play experience/knowledge of past experiences and how your dice are rolling.
I almost never math hammer in games! I dont need to because it slows me down and I already know how I will do based on past games. In a timed tourney setting I dont have the time to spare to think. You should be actively reacting and moving without needing to math hammer. Another reason is math hammer may show an even result or a non favorable result and change your decision. You need to take risks to win and make the appropriate choices. Your opponent may be rolling bad and you can take advantage of that situation. Also your opponent might make a mistake if you went for it. You would never know that if you held back due to a poor math hammer result.
Before you start crunching numbers in your next game ask yourself "do I already know the outcome?" you can save so much time and fear not using math hammer in games.