Most people I know play blast weapons incorrectly. Most people say I want to fire the missile at that unit using frag and then they measure and find that the unit is just within range so they place the template at the front of the unit. This is wrong.
Blast templates are placed prior to ANY measuring range. You then measure and if the center of the blast is outside the range of the weapon it is an automatic miss. This also applies when using a Grek Knight heavy incinerator or a hellhound inferno cannon. You place the flame template, then measure to make sure the small end is within '12.