Effort Estimate for migrating applications from HP Alpha to HP Itanium

We need to migrate our application which is currently running on HP Alpha using OpenVMS. Application are clustered and it is a site based installations. There is only one version of each application which is used for multiple site.

I wonder how do we really calculate the effort needed of migrating these applications from HP Alpha to HP Itanium or Linux.

Is there any standard estimation technique which should be use for calculating resource requirement, schedule and cost of the total migration?