Antler will work for a tip but most screw tips you see have Horn tips. You have to be careful with antler, if it is dry or Pithy it will not thread well, you need a good solid piece of Antler. With internal threads as you are doing, I would start the threaded part of the tip at about 1 1/4", taper it so the die will start easier, once threaded cut it down to about 1/2-3/4". Slightly taper/round the lip so it will start in to the horn easier. Once threads are cut on both you can trim the Horn to get a good fit, put a little Magic marker on the edge of the tip screw it in and remove the Black marks, go slow. Goes a little better if you work the horn down to final or close to final shape. So far it looks good but I would reconsider the Antler, leave it for a future project like a Southern Banded horn. Are you thinking about a collar? Do you have a particular style horn in mind? That will help you determine the length.
My best advice on Screw Tips would be to get a copy of Art DeCamp's wonderful book:
Pennsylvania : Horns of the Trade" Screw-tip Powder Horns and Their Architecure
Tim