The only hint I can offer is have your pieces at least a half inch all around bigger then the unit (including the seam allowances). I know it seems like a waste of fabric but it is the only thing that keeps me from shorting myself. Sometimes I go even bigger. I will measure the individual unit on the pattern at it's longest point and widest point then add an inch and cut my fabric to that size. I still sometimes manage to short myself. But sometimes I get really lucky and when I cut it down to prep for my next unit I may have a big enough sized piece for another unit of the same color.