use case is diagram in UML to provide description on project main functions and for what objectives they might used, excel has special template to support project estimation of development please check out this link it probably helps

http://www.codeproject.com/Articles/9913/Project-Estimation-with-Use-Case-Points

SparxSystems.com.au has one - you need to pay for it. The UCP method is described here and it is easy to build you work spreadsheet from it http://www.methodsandtools.com/archive/archive.php?id=25

Be aware the UCP only provides developer effort and you need to add project manager, tester, config mgt, etc effort for a whole project effort estimate. I apply these as overheads to the developer effort.

Also, an estimate is very sensitive to the state of requirements. Take a look at the "cone of uncertainty" to see what I mean. The smaller effort estimate against each item of functional scope the less the risk of misinterpretation which leads to less risk that implementation effort will exceed estimates.

some templates I used earlier.

hope this helps.

thanks a lot for your valuable suggestions.

Use-case-based effort estimation database (UCEEDB)

http://www.se.cs.put.poznan.pl/knowledge-base/software-projects-database/use-case-based-effort-estimation/?searchterm=None