be aware that the sql memory options are for buffer cache and not the other various memory pools. I generally never use lock pages in memory as this allows you to see how memory is actually being utilised. Note that Tempdb may use as much memory or greater than your database(s)

As with any such scenario you have to monitor and trend to best tune the resources to the demands of the server. I'm assuming this is all x64 so watch your other memory pools.

My advice - take a estimate at what you think is fair - then monitor ( record to a table ) and adjust to suit. Taking pot luck might not be the best choice!

Although I have no affiliation to SQL Diagnostic Manager I'd suggest this as a useful tool to assist.