- Domain controllers per domain per site needed
- Global Catalog servers per domain per site needed
- CPUs per machine and type of CPU needed
- Disks needed for Active Directory data storage
- Amount of memory required
- Network bandwidth utilization
- Domain database size
- Global catalog database size
- Intersite replication bandwidth required
Its modelling is only as good as the data you enter. Do due diligence on the data entry and you should get out decent estimates. There are many options depending on your organization structure and application needs. Let me know if you use this tool and find it valuable.