bblast wrote: there is simply no scope for any lapse in concentration on test day.

We're humans and even if you try to think that you won't allow your concentration to lapse on the test day, it will waver. Accept that and relax .

So you have to be perfect.

I think it is futile to think that you will be perfect on the exam. And trust me ,if you even think of being "perfect" on the exam, you'll be doing more harm than good to yourself . This unnecessarily is going to build up pressure from now itself and you surely don't wanna do that.

Even an initial bad 5 minute spell will ruin the score and all the hard work.

Not necessarily. I remember that I got nervous on the Quant section in the beginning and I was just plain "anxious" and took more time to solve even easy questions because of sheer anxiety. However, after sometime, I just happened to observe myself why I am being anxious and then gradually I was in control. I think by having thoughts like "Initial 5 minutes will ruin the score" you're building up the anxiety which is gonna be detrimental on the D-day. I know to be calm somedays before the exam is easier said than done ,but at least you can try

Past test takers, any final tips for me ?

Just enjoy,laugh a lot ,relax ,unwind now as you need to be in your optimum performance level on the D-day-these tips are worthier than any other tip I can give you for the exam.
All the best! Do let us know about your score

Thanks mate,
Your words have been a fuel to me ever since last month when I began the prep.

I've reduced study hours now. Just doing some random SC's-CR's and online math questions to stay in the "Mood".