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

O < C

A Short Black Candle is also a Short Candle. The following formula checks for the body to be more than 50% shorter 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 < AVG(ABS(O - C), 15) / 2

Combining these formulas results in the following.

O < C AND C - O < AVG(ABS(O - C), 15) / 2