What is the correct way of calculating the "average" price based on the "Open,High,Low,Close" daily trade fields?

You can't calculate an actual average from that data. An average is the sum of all values divided by the count of values. If you are just making a trend-line or time series, just use Open or Close.

I would use A = (low + high)/2. I am not aware of this being a common technical indicator.

In regard to common practices "VWAP" refers to any algorithm that calculates a volume weighted average price. I have seen a few references that define simple VWAP = volume x (low + high + close)/3.

https://www.investopedia.com/ask/answers/031115/what-common-strategy-traders-implement-when-using-volume-weighted-average-price-vwap.asp

This simple algorithm is used for a period which aggregates tick data.

If all tick data is available for a period then VWAP would not be estimated from low, high, and close data, it would be calculated directly from the actual trade price and volume for each trade.

One application of VWAP is to buy or sell a large stock position over time. Instead of trying to buy or sell large blocks of shares all at once, the large order is broken into smaller trades that execute close to the VWAP. The theory is that this will produce the best execution.

You cannot obtain that information with the data provided.

You would need periodic samples throughout the day.