A Long White Candle is first off a White Candle which opens below the close.

O < C

A Long White Candle is also a Long Candle. The following formula checks for the body to be more than 50% longer than the mean body size over the most recent 15 candles. It can use C - O instead of ABS(O - C) at the beginning of the formula because we know O < C.

C - O > 3 * AVG(ABS(O - C), 15) / 2

That is actually the final formula because if it is true, then O < C also has to be true.