help figuring out what % of protein in a custom mix?

I was wondering if someone could tell me how to figure out the % of protein in a custom mix feed.  I dont have one put together yet but would like to try and figure out how to do this.  When we get feed we normally only get about 1000 lbs at a time to keep it fresh.

Also for a grower ration you want to keep the protein somewhere between 12 and 14 % is that also correct?

Thank you for your help on this topic we have never tried putting our own ration together but thinking about it for this coming year.  We have only be in showing for about 3 years now and we try to find out as much as we can by asking so I hope that you all don't get upset if we continue.

Thanks again in advance....

I almost forgot when do you normally switch from a grower to a finisher.

oats 12 percent, corn 9,  calf manna or similar 25,  cotton seed hulls 3.  100 lbs oats will get you 12 lbs protien  100 corn  9. so lbs protien per ingredient divided bytotal wt of mix and there it is

Heres the program I use from OSU.  After you play with it a bit its pretty neat.

Ron
<cowboy>
http://www.ansi.okstate.edu/software/