The issue is chain tension. If it's too loose it slaps around a lot and can come derailed easily. If its too tight the bike won't pedal smoothly. The T1-T2 thing is just an objective way to measure how much slack is in the chain.

In real life I just gradually snug up the eccentric until I start to feel some resistance or vibration when I turn the crank by hand - then I back it off until it goes away.