Therm,
You won't have to worry much about how full your case is. What ever you load they will be PLENTY full and you might as well order powder by the 8lb jug. Your headspace, once you have fired the rounds in your rifle, you are good to go for neck sizing only. No headspace worries! I have a Criterion 28in varmint barrel on mine with a short throat. Mine is also mag fed so length is limited for me. A factory rifle has a very long throat and to get off the lands by .01 you would have to seat very long. You will enjoy it! Have some fun!