Based on months count increment leave value

Hi,

Employee annual leave limit per years is 30. Per month 2.5 is added as a annual leave balance based on his joining date.

For example say employee completed 3 months his annual leave balance count should be 7.5.

1st month - 2.5
2nd month - 5
3rd month - 7.5

How to achieve this using sql / .net


Regards,
Ayesha