Guys, I trully believe that unless before the G-day you slept 3 hours or had your worst hangover ever or was starving for 5 days, your performance won't be greatly affected. Leave all these 'tuna fish' diets, 'supermegaenergybrain' bards, and 'endurance' drinks for marketing departments of food producers. You need to build resources/confidence in yourself, not to lease it from 3rd parties (including many of prep companies
)
What you really need is A SET OF RIGID MENTAL SAFETYCHECKS (OR RED FLAGS FOR SC) FOR THE MAJORITY OF QUESTIONS. For example, for Quant/DS you need to have 'ZIP' embedded into your brains. ZIP stays for Zero, Integer, Positive and shall remind you not to assume that variable in your DS question is Zero, Integer, or Positive! That single mnemo-reminded saved me from dozens of incorrect answers. In SC you need to bear in mind reg/question marks around words 'HAD'? 'BEING'? and a couple of others.
That stuff, not meditation while eating energy tuna bars will really make a difference on a G-Day.
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P.S. On a practical note, if you pass all 25
GMATClub tests and analyze your mistakes, preferable two times, I doubt you'll have issues with Quant timing and scoring below Q42-44 on the real test.