Hi Niladri,
The system is allowing you to avail 8.5 hours because you might have copied the existing Leave Types.
Check below table to make the changes.
1) In Table T554S. Keep the Maximum Duration as 0000 ( This will restrict you to availe full day Leave)
2) In Table V_T559R Rounding Rule ( Apply a rule >0.1 to 0.99 should be 0.5).
Regards,
Niranjan Ghosh