Thanks for the follow up Rishi.
I was able to calculate Stack. Our Solman system already has required SP level.
I followed below steps:
Login to Solman system.
Goto to DSWP
Goto Change Management
Maintainance Optimizer
Select New Maintainance Optimizer
Select the Source System SAP R/3 4.7
And Select Upgrade button
Select the Target system ERP 6.0 EHP6
And follow rest of the steps for calculating stack and download.
Thanks,
Srikanth