Just move the greater numbers to the top of the else-if tree:
Code:
if(DmgDone <= 30)
{
IgniteEntity(client,3.0);
}else if (DmgDone > 71)
{
IgniteEntity(client,12.0);
}else if(DmgDone > 51)
{
IgniteEntity(client,9.0);
}else if(DmgDone > 31)
{
IgniteEntity(client,6.0);
}