50% retracement is often also true during INTRADAY movement - DESK FOR INTRADAY trading

While moving averages and RSI (in second place) could give the trigger point for the change of trend, this rule gives you the "stop" level for closing intraday position.

Calculations need not to be accurate.

So, the 50% is true both for daily and intraday graphs.

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For INTRADAY trading the above desk is sufficient: just EMAs, Bollinger Bands (but NOT with top spin up and down), 50% retracement, RSI, 2-hours rule and MACD...
the major component in trading is your intuition (rather than a precise technical analisys as end-of-day or daily graph)

However, a lecture of book of R. Di Lorenzo,

Come guadagnare in borsa con il trading intraday

is advised (but not necessary, beside because moving averages are already inserted), in order to keep inspiration for introducing new and more reactive (...) indicators.

Any source