customizing use case metrics

[albena]

I would like to generate a use case based project estimate by using the EA Use Case metrics as explained here: http://www.sparxsystems.com/resources/ucmetrics.html

How can I customize the calculation to use a custom made use case tag value called for example "Estimate". In addition, how can I structurally add a new tag value to all my use case items?

Thanx for your help.
Greetings,
Albena