Calculate memory need for SQL Server

SQL Max Memory = TotalPhyMem - (NumOfSQLThreads * ThreadStackSize) - (1GB * CEILING(NumOfCores/4)) - OS Reserved

NumOfSQLThreads = 256 + (NumOfProcessors*- 4) * 8 (* If NumOfProcessors > 4, else 0)
ThreadStackSize = 2MB on x64 or 4 MB on 64-bit (IA64)
OS Reserved = 20% of total ram for under if system has 15GB. 12.5% for over 20GB