In order to use your ActionForm bean in a JSP scriptlet, you have to retrieve it from the scope in which it is saved. So, if myActionForm is in request scope, the code to retrieve it would be:
<%
MyActionForm myActionForm = (MyActionForm)request.getAttribute("myActionForm");
%>
Once you have this reference to the bean, you can call any of it's methods, just as you would in regular
Java.
Another alternative might be to put the complex calculation into a getter method in the ActionForm bean (e.g. getMyComplexData()). If you do this, you can access the results of the calculation with <bean:write name="myActionForm" property="myComplexData" />