The rule is to take the currency pair's average daily range (ADR) and divide by a number between 2-20.
The smaller the divisor number, the larger the trend you want to identify. Smaller divisor number also identify But typically, 1/3 to 1/10th the ADR is plenty for most traders.
So let's say that the EURUSD's ADR is 200.0 pips. You want to identify a medium length intraday trend. Let's try 1/6th, or 200.0/6 = 33.3 pips. You would then set the APAMI moveComplete input to 333 (33.3 pips for 5 digit broker) and observe. If you want to try a different length, either change the indicator properties settings, or open a new chart and test a new length setting with a separate instance of the indicator.