The normal rule is:
HST: add 7/8 to finished size block.
QST: add 1 1/4" then make 2 diagonal lines
Here's a site that makes making HST's easy peasy:
http://www.blockcentral.com/a-halfsq...glepaper.shtml
Here's a site is you need a different size you can enlarge or reduce with your printer:
http://www.thequiltedsnail.com/scale.html
Hope this helps out.