Hi Franco,
Do you use TS7 or TS2000i ?
The problem with creating your own Trailing Stop as a strategy is that Strategies are only run at the close of each bar. For example if you have a 15mn BarChart, your custom Trailing Stop will not be recalculated during this intervall but only every 15mn. Can be a little bit damaging for your finances!!! TradeStation proprietary Stops are calculated on a tick by tick basis.
However, if you work with 1mn barChart, such custom Trailing Stop would work. I have an example in EasyLanguage for TS7 which needs the use of 2 functions. If you're interested le me know and I post it to this thread. Also, you can use the Trailing Stop strategies provided by TradeStation (they are in TS7, I gues they are in TS2000i). They are called Dollar Trailing and Percent Trailing.