Welcome to the Asana Community Forum @Petr_Havlik 
In this case I recommend to have the goal progress calculated based on milestones in the project (you can convert the specific tasks to milestones) or you multi-home all the tasks you need (meaning to add all the tasks to a 2nd project which you can do very fast in list view by bulk editing) and then set up the goal for the specific new project.
Also here is a feedback request you might be interested in voting.
This might be interesting as well.