Affectation percentage versus Assignment Rate versus Resources' FTE

Hi!

I've been banging my head against a wall for some time with Affectation percentage and Assignment rate and still can't figure out how they work (specially together).
I'll try my best to give you the picture of my situation so you can help me (or at least try ) figure out how is the best way to do the planning.

I have a resource who will work in a project for 4 hours a day (half of his daily capacity) so, I set his affectation to 50%.
When I did the calculation of the project, the resource's workload was set for 8 hours a day for all activities where he was assigned.
That's not what I expected, so I changed the affectation back to 100% and set the assignment rate to 50%.
Another calculation and now the workload assigned was corrected on each activity, but there were always two activities running at the same time (and I understand why, but again, that's not what I expected).
So I returned to affectation and set it again to 50%... another calculation and no changes... still get 2 activities running at the same time, causing the daily workload of the resource to fill 8 hours instead of the 4 hours I wanted.
So I searched through the system and found FTE in the resource form. That solved the workload problem, but I can't keep the resource working 0,5 FTE for the eternity.

Any idea how to solve this in another way?

Thanks and regards!

ps.: planning mode "regular in half days" won't solve the problem, 'cause that's more resources planned on the same activity and they have no restriction of daily capacity.