Planner-list Planner resources/tasks: How to handle people working part-time only?

Dear all,

I have started to use Planner just recently and I try to set up a 
project for which one person 50% of his time. I tried to accommodate 
that by setting the working hours in his calendar to 4 hours per day. 
Still, if I create a task with fixed duration, that takes 8 hours and 
should be completed within 2 days the person is assigned with 50% of his 
time instead of 100%. How can I get this consistent, i.e. have planner 
take into account that the work day length is not 8h for that resource? 
Or is there any other possibility to handle part-time workers?

Thanks a lot and best wishes
Sabine