Hi, Juan!

OnEntry/OnExit works like events!
If you put "MyFunction()" into OnEntry of page header, "MyFunction()" is fired if the report processing starts the processing of page header.
"MyFunction()" must exist and must be in a reachable scope (i.e. in a SET PROC procedure file).

In "MyFunction()" you can do what you want, i.e. calculate value or other stuff!

BTW: If you want to deal with this feature be aware of multiple firing if the user skips forward/backward in preview mode!