Hi,

There is no planning mode that will ensure this so specific planning.
Maybe with a planning mode 'regular in half days', for a task with 40 hours assigned, you'll get something apprimatively fitting.
The best way to get exactly what you expect is to directly enter the real work in the real work allocation screen for this task.

I have a 3 days duration activity which in regular planning (as soon as possible) is planned correctly (3 days), but when I change the planning to Regular in Half Days, it remains with a 3 days duration instead of 6. I'm attaching the activity'ws configuration screen.