I would use a 500ml yogurt container to determine the space. Use it to get an idea of the area around the frag, and you get a rough idea of the end size before you get things touching. This assumes uniform growth, which seldom happens. Stags tend to go up, so you can space closer. With me, most of my frags came from my colonies, so I have a good idea what the end colony would look like, shape wise.
But, and I recall your new(ish) to SPS, you'll end up collecting more and more until you're out of room and too close
We all do lol