On Tue, 19 Jun 2012 09:23:44 -0700 (PDT),
wrote:

I estimate that I will be needing about 85 bags of 90 lbs.

Is my reasoning sound and am I missing anything.

IMO, anyone wanting to mix 85 bags of concrete is not of sound mind
and is not reasoning at all. Get reddi-mix delivered and have help
standing by.