Indicator for Market deviation of a range

I'll spare you all of the gobbledygook about how people analyze and use the Stochastic Oscillator. However, it does depict the position of the underlying in relation to the past "X" days and would clearly show deviation from the range.

If you have access to analytical software, you could also calculate the average daily price change and then screen for time periods where price change exceeds that amount by whatever number amuses you.

Or you could simply look at the WSJ intraday list of largest price and percent movers and then look at a chart.

I understand that you want to calculate things on your own, but it is better not to reinvent the wheel.

People have created technical analysis libraries based on the renowned https://ta-lib.org/, such as https://github.com/mrjbq7/ta-lib for Python that contains 150+ indicators.

The simplest indicator that serves your purpose is probably Bollinger Bands.