SQL Server & RAM

I have a 24GB RAM Windows Server 2008 R2. The plan for this server is just for SQL Server with multiple instances. Some of these instances will have SSRS and SSIS.

1) Is the rule-of-thumb to allocate 35-40% of RAM to OS?

2) How do I split up the available remaining RAM to Maximum server memoryamongst the instances initially; not knowing the usage statistics, etc.

3) Do I continue to adjust the Maximum server memory for each instances as we go?

Thanks