Simply touch the timer or press your Spacebar to start and stop! I'd recommend using 15 seconds inspection time, as that is what official Rubik's Cube competitions give you to look at the cube before starting your solve. And if we're going to care about official Rubik's Cube rules (who doesn't), then WCA regulation 4d1 says that cubes should be scrambled starting with the white face on top and the green face on the front.

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