Post width and margin issues

I can’t for the life of me seem to get my post width the way I want… The default post width for my blog is 900px but I find at that width it disrupts the flow of reading. When I set a new width, say 750px, my margins screw up and the post box becomes off center. I’ve tried tweaking the margins, but I can’t seem to find appropriate values to make it work.

These are the default values in my stylesheet:
.post .single {margin: 80px 0 250px; padding: 0; background-color: #fff; box-shadow: 0 1px 1px 0 #999; -moz-box-shadow: 0 1px 1px 0 #999; -webkit-box-shadow: 0 1px 1px 0 #999;}

Here‘s an post example. Any assistance would be greatly appreciated.