It's easier if you've drilled the hole in the horn yourself, because you can then use the same drill to make a hole in a scrap of hardwood, cut a slot into it to take some sand paper and use that to shape the bow tips giving a perfect fit, in the manner of a pencil sharpener.
Maybe press some moddelling clay into one of the nocks, when you pull it out it will show the shape you need?
A sprinkle of powdered chalk in the nock will help show the high spots on the bow tip as you push it in and rotate it, keep sanding off the high spots until you get a pretty even chalk mark.
You'll know when it's right, it will fit snug and won't wobble. Check the depth of insertion too, you want to make sure you are all the way in.
Del
(BTW I use a good quality rapid epoxy for fitting)