i'm not sure what the formula would be but i imagine its the same formula as a 1/4 square triangle because thats basically what you end up with.
you take a square and fold it into a 1/4 square triangle.
you can make continuous prairie points at any size the basic rules are as follows.
cut the width of the strip to the size on 1 quilt side plus 0.5 for good measure.
the the length of the strip is 2 times the size of your square - if you're folding a 4 inch square you want 8 inches if its a 5 inch square you want 10 inches - etc.
then you cut the squares on one side of the strip. then for the other side you make the first cut at half the square amount and then all remaining cuts are at the full cut.
so if you have 4 inch cuts the first cut on the other side would be at 2 and the remaining cuts would be at 4.
if you are doing 5 inch cuts the first cut would be at 2.5 and all remaining cuts would be at 5 inches.
i like the 4 inch square if you're going to be pointing them in.
after having done this i would do 5 inch squares if i plan on pointing them out because a good chunk is lost in the binding then to flip them out and over the binding it might be best to just do 5 inches to point them out.
if i were going to be doing them within a quilt pointing in (instead of on the edge) i plan on doing those cuts at 3 inches to make a nice little internal border type element - these are the ones that i plan to do piping on - to really draw the eye to the center.
i really like that they are 3 dimensional and add a movable element to a quilt.
i'm going to be playing with prairie points for a long time
:D