I think the "preferred" way to make HST is to add 7/8" to the FINISHED size of the block you want. So you want to start with 6 7/8" block. Sew line corner to corner, stitch 1/4" from center line, cut and wala! 2 HST blocks measuring 6 1/2" unfinished, 6" finished. I actually use a ruler by Cindy Casciato called "Migicle Triangle Method Tool". With this tool you cut fabric +1" instead of + 7/8", mark lines, sew and cut. Block is perfect every time since the tool allows for the 1/8" extra. Personally I do not like to over make block and then try to center and cut down.