Thanks to all,
my problem is that I need to know the correct value (height) of Arrangement3, but for my understanding (now) it is not possible directly (see the last two blocks in my when do statement). I have to add up all the components and then subtract them from the total length.
You have to get the height of the status and title bars to eliminate them from the equation, then a clock timer is needed to give the arrangement heights time to catch up with themselves. I used a 200ms timer.