Memory utilization for multiple instance (SQL server 2008R2 standard edition)
You can leave it to default for all instances because since it is SQL Server 2008 R2 standard edition it will not use more than 64 GB and you ahve total of 256 GB RAM so you are on safer side. Please also note that 64GB limitation is for Database Engine only other components like SSIS,SSAS,SSRS does not falls under this restriction and yes they can also consume good amount of memory.