maximum allocate RAM for MS SQL Server
Calculation is something which you need to analyse an figure out with some calculation on basis of that. Kindly go through the below discussion which will help you out in deciding what values you can assign:

What is a deterministic method for evaluating a sensible buffer pool size?

Also, would recommend to go through below article for indepth explanation:

http://sqlblog.com/blogs/jonathan_kehayias/archive/2009/07/07/understanding-the-vas-reservation-aka-memtoleave-in-sql-server.aspx