Planning project with fix time estimate and end date

Hi.

I am struggling with correct definition of our project I am importing to ProjeQtOr.
If they are not a bug solving projects, all our projects are defined in similar way - all have fixed effort estimate and fixed end date.
Assigned resources always have more than one overlapping projects active with not defined start dates but end dates.

Up to now I was not able to define project in correct way so they are planned with preserving both fixed values, the effort estimate  and the end date. What ever I do, change or select, at least one of those values is changed. In most of the cases, when I change project parameter, its recalculation change planned effort estimate from defined one to much higher one covering whole period from start date to planned end date.

I know I am doing something wrong, our project definitions are nothing new and strange but am unable to find what is that .
Any suggestion and tip is very welcome.
 

Yes, I would gladly attend a training session and I am waiting for some of them in English.

Super, will try not to miss it .