may want to read
http://www.elitetrader.com/vb/showthread.php?s=&threadid=11388
http://www.elitetrader.com/vb/showthread.php?s=&threadid=29910
it also seems that there maybe more important considerations than margin interest paid
you may want to have a closer look at
https://www.interactivebrokers.com/html/tools/freelanceTools.html
TradeStation provides no API enabling similar tools to be developed ...
as I understand margin interest is applied only to overnight borrowing (losses too ...)
otherwise gains/losses can be transferred using their FX system as triggered
there are multiple other threads on this topic
http://www.elitetrader.com/vb/showthread.php?s=&threadid=16242
http://www.elitetrader.com/vb/showthread.php?s=&threadid=14340
http://www.elitetrader.com/vb/showthread.php?s=&threadid=1871...