the 1/4" seam allowance also applies to each unit within the block.
A finished 12" block which is not joined to other blocks is 12 1/2".
Reduce finished 12" (not 12 1/2) to 6". But you have to add 1/4" to each component of the block and the outside seams.
A triangle in a sqare would be plus 1/2" for the outside seams, no extra width-wise, but plus 3/8" plus 3/8" for both sides of the triangle height-wise. (triangle is pointing up or down)
A pinwheel would be plus 1/2" outer SA plus 1/2" inner SA both directions, ditto a sq in a sq.