OK.
To all of you: Thank you very much.
I have one remaining problem:
At the moment I really don't get the math for the monsters.
There may be tables and so on, but that isn't really helpful for the pregenerated monsters.
To me it looks like a lot of work and doesnt fit very well with the pregenerated encounters.
If I halve the damage, then a Group of 5 PCs (which is the status at the moment) would fight to easy. Yeah I could give lower XP, but now they are level 7.
If I'm honest I need a practical solution for the encounters without too many changes.
I also found a tool, which is free and works an Mac and Windows:
4eTurnTracker D&D 4e Encounter Tracker
Additionally I will think about cutting out some encounters. But not on the Rule, that I need to cut out 1/5 encounters for a 4 PC Group, but based on if the encounter makes sense or not. However I will reward my PCs with the XP from the cutted out encounter, so they will leveling at the right rate.