SQL Server Max memory limit
SQL Server's memory manager is designed to keep as much data cached in memory as it can in order to speed up access for incoming queries.


In order to work successfully with other processes on the machine,
SQL Server will release memory when a low- memory notification is
sent by Windows.
The memory may be committed by the SQL Server process, but it will be
released if another application requests enough to trip the
low-memory notification.


Before you proceed with setting up the MAX and MIN server memory config setting would recommend you to go through this great write up by Jonathan How much memory does my SQL Server actually need?


  Ultimately what I require is the OS utilization should not go above
  90% which is why I kept the max memory to 28GB


If you are changing above just to settle down the threshold, sounds may not be the perfect reason you should be cutting down the memory what SQL needs.

However, the first thing you will need to analyse what process that are running at OS level and for how long that needs the amount of memory you are looking for, because SQL will use what has been assigned.

Baseline the server accordingly and see if you really face OS memory pressure:

Also see here  and here.

In addition there is this excellent article from Shanky : SQL Server Memory and Troubleshooting which should get you going.