Allowing for correct amount of movement in table top

If you have a table top and want to add a border you basically have to viable options that I know of, bread board ends or a spline and mitred corners.

Now what I can't get me head around is where you assume your start point is? For example do you assume your starting at minimum size and allow for full expansion, take it as being at mid point, etc?

Rule of thumb I have read on here seems to equate to 1% expansion over the width of a board. I'm also thinking in terms of hard woods. Blackwood, Jarrah, oak etc.