On Point instructions from quiltbug.com

Piecing a Quilt on Point

A quilt that is set on point is almost always more interesting than one done with a straight set.  But it can be tricky to piece, especially if you are creating your own design.  The first question always is: how big do I cut the side and end triangles?
The side setting triangles on the quilt above are in yellow.  To cut them so that the bias is on the inside and the straight of grain along the edge, you must start with an oversize square and cut that one on an X. The diagram to the left illustrates this. The formula for this is: Size of the finished on-point block times 1.414, then add 1 1/4 inches.
The corner setting triangles (the purple ones in the illustration above) are also cut so the bias is on the inside and the straight of grain is along the edges, but since they include a corner of the quilt, they are cut differently.   These start with a square which is cut on the diagonal. The formula for this is: size of the finished block divided by 1.414 plus 7/8 inch (.875 if you are using a calculator.)
Here is a quick reference chart:
(these numbers have been rounded up slightly)

Finished size of block	Size to cut square for setting triangle	Size to cut square for corner triangle
4	7	3 3/4
5	8 1/2	4 1/2
6	9 3/4	5 1/4
7	11 1/4	6
8	12 3/4	6 3/4
9	14	7 1/2
10	15 1/2	8
11	17	8 3/4
12	18 1/4	9 1/2
13	19 3/4	10 1/4
14	21	11
15	22 1/2	11 3/4
16	24	12 1/4
17	25 1/4	13
18	26 3/4	13 3/4
19	28 1/4	14 1/2
20	29 1/2	15 1/4

  Could you use this formula to set a block on point?  Yes, you could use the purple formula, or try this one: Size of the block multiplied by itself divided in two.  The square root of this number would be the size of the square you would need to cut on the diagonal.
Conversions from inches to calculator:

1/8	.12	5/8	.62
1/4	.25	3/4	.75
3/8	.37	7/8	.875
1/2	.5	1	1

Now, how do you actually piece this quilt?  You sew it sideways in rows, like this: