That memory is not an issue. SQL Server will automatically use all the memory it has available to it. Always. Every time. So you don't have to worry about whether or not it's using the memory, it is. All of it.

Instead, you need to worry about HOW it is using that memory. Is it able to read from memory for most queries? Great. Win? Or, does it have to go to the disk for lots of queries, moving their information into memory, throwing out old information, only to do that again and again? That's a problem.

Instead of focusing on memory percentage, you need to be looking at your wait statistics through sys.dm_os_wait_stats and queuing through the various memory and disk queue indicators. That will tell you if you have problems and where those problems may be. You also have to look at the queries, which ones are long running, called frequently, or just use lots of resources. Tune the problematic queries.

That's how you go about monitoring. Don't get hung on a single number, percentage of memory, especially when that number is actually very unimportant.