On a CPU task (LHC Atlas VirtualBox task) that is set to use 3 CPU cores always shows 100% usage after the initialization of the task is over.
Is on my to-do list, but it may take a while.
The usage value is normally not used, it's more an indication for the user.
Setting the usage to 1.1 wil most likely change nothing but the client seems to divide the value by 2. That is correct BOINC can't assign 1.1 CPU, its always the whole core 1, 2, 3.
The CPU usage is simple the number of CPU cycles the task reports in a certain time frame, so it shouldn't change at all.
If you want to allocate a certain portion to BOINC tasks go to BOINC Settings and in Processor set "On multiprocessor....." to e.g. 50% to use half of the CPU cores.
With a Virtual BOX anything can go wrong, but I will try to test it.