j4
Here's a link to a ration formulation spreadsheet I use and it has proven to be quite accurate. If you put your ingredient cost into the feed list page it will give you the projected cost of gain.
http://www.ansi.okstate.edu/software/
For a 600# steer 8#/day of your ration would only gain .32/day whereas 10# would get .89 adg and 11# would get you 1.16 adg.
What this spreadsheet has taught me is the more feed the lower the cost of gain and higher adg. One of the downsides to heavy feeding is getting them too fleshy and hurting their grass gain. If i were in this situation I would target their adg to something less than their expected adg on grass. If they are fed too heavy now they could "back up" when turned out on grass, while too limited feeding will run up their feeding cost of gain. It's a balancing act that depends on feed costs and the desired end weight/date.
My best guess would be to feed 10-11#/day until turnout.
Just my 2 cents worth.