fiber percentages in feed

I know that fiber is crucial in show cattle feed to help give more spring and depth of rib.  We only have 30 days to our county fair and we could use a little more of each.  What is a minimum % of fiber needed to achieve this goal.  Also how many pounds of a fiber source should be mixed into feed.  My friend recommended purina Pre-con.  It is 26% fiber.  Would this be sufficent or to achieve my goal should the % be higher.  This topic allways confuses me.  Thanks for any input.

We have had good much mixing Purina depth charge into our finishing ration

Free choice medium coarse grass hay and we include about 12% dry beet pulp in our show feed.