Increasing enough RAM for OS from Total Physical RAM
(Full disclosure: I created the Max Server Memory Matrix and associated script that you are referring to in your question.)

Per this excellent blog post, which contains a detailed explanation of the issue you're experiencing, you'll find this little quote:


  The recommended Page reads/sec value should be under 90. Higher values
  indicate insufficient memory and indexing issues.


Without knowing too much about your environment, as you did not give us very much to go on, you may benefit from more RAM. For instance, your database is 400GB in size, and data access patterns require large scans to access lots of data.

On the other hand, your indexes might need attention. For instance, your database is 400GB in size, and data access patterns require large scans, when in fact you only need to read one or two columns in that table.

Instead of worrying about buying hardware, perhaps it would benefit you to start with the basics:


Have you analysed the missing index DMVs to identify possible new indexes?
Have you considered using filtered indexes?
Have you rewritten your queries to make better choices with data?


The point is, there's a lot you can do before considering purchasing more RAM. Figure out what your queries are doing, and help them make better life choices.

Brent Ozar has just open-sourced his course on how the database engine works, and I reckon you would benefit from that, as well as many other free online resources.