You find the appropriate bullet for 1000 yd and then work up the tightest groups you can at 100. In load development, you want to test accuracy of the load, not your ability to read wind, mirage, etc. You will discocover that 1/2 minuute group at 100 is not 5" group at 1000.
Just because a load will shoot at 100 does not mean it will shoot at 1000. You can be guaranteed that if a load wont shoot at 100 it wont shoot at 1000. A rifle needs to be a 1/4 minute 100 yd rifle to shoot decent at 1000. I have 13 1000 yd rifles. .284 Winchester is the way to go. Had I built the .284 first, I would have 1 1000 yd rifle. Of course Berger did not make the bullet that makes the .284 shine until recently either.