See Paul Ham's book HIROSHIMA, NAGASAKI, St. Martin's Press, August 2014, available now in paper back.
APPENDIX 9: "Pentagon's Estimated Bomb Requirements for Destruction of Russian Strategic Areas," SEPTEMBER 1945.
The chart lists Russian/Soviet cities and the "No. of Bombs" needed for the destruction of each. For example, Moscow: 6, Leningrad 6, etc. for a total of 66 Cities with 204 Bombs needed to destroy them.
What is striking is the date: SEPTEMBER, 1945.