Runic Power priority for me typically has been:
Rune Strike > [Unholy Blight/Frost Strike] > Death Coil
You pretty much shouldn't be using Death Coil ever as frost, unless you're picking up an add at range with it. I use Death Coil as Unholy only when I don't need to refresh Unholy Blight. But always try to keep at least 20RP (remember, RS now only uses RP when the swing actually occurs, not when you first trigger the ability) for Rune Strike.
Luck is what happens when preparation meets opportunity. -Seneca
Everyone marvels at a square egg, but only the chicken understands the PAIN.