Is there a standard formula to calculate the optimal resource required by SQL Server base on the Ram size of the server
The link that you pointed out gives a good starting number, which in most cases is sufficient enough as a good configuration.

I would suggest you to leave min memory as DEFAULT and adjust the max memory.

Also, best is to baseline your database server usage during your full business cycle as that will give you the best number based on your workload using below PERFMON counters :


SQL Server:Buffer Manager\Page Life Expectancy 
SQL Server:Buffer Manager\Page reads/sec 
Physical Disk\Disk Reads/sec
Memory\Available Mbytes
SQL Server: Memory Manager - Total Server Memory
SQL Server: Memory Manager - Target Server Memory


Note: If you are going to use any formula or online calculator to calculate SQL Server memory configuration then best is to read Beaware of Wow… An online calculator to misconfigure your SQL Server memory ! - by Jonathan Kehayias  first.

Some good references :

How much memory does my SQL Server actually need? - by Jonathan Kehayias