I think that to some extent reading signs from candle burning is situational and individual. Some may disagree with this, but that has been my personal experience. Use your intuition and do (or have a reader do) a reading on the work. Use Miss Cat's reference page (http://www.luckymojo.com/candlemagicdivination.html
) as a guideline, but I would suggest that ultimately you have to come up with your own system of interpretation based upon trial and error and the observation of what signs you see versus how your work plays out in the manifest world. As in dream interpretation or the like, there are no universals: the same symbol in two different minds or cultural contexts can mean two completely different things, and that doesn't necessarily mean that one is wrong and the other right.
That being said, I think it is generally accepted that a clean, even or "traceless" burn is usually optimal. Here's what Miss Cat says about it on the page linked above: "This means things will go well with the spell or blessing and that one will most likely get what one wishes for. If a glass encased vigil or novena candle burns and leaves no marks on the glass, that is best. If a free-standing candle leaves little or no residue, that is best."
On the other hand, a run of wax droplets down the side of a candle "...denote[s] that someone will cry before the spell succeeds. If the tears persist and form a tall column that never melts down, the sorrow will be of long effect." The shape you mentioned of the "dagger" on the side of the jar could certainly fall into this category.
My personal reading of the big, sudden pouring of wax down the side of a jar was that it was essentially the opposite of a normal, even-burning candle (which would have been the most desirable), and therefore indicated that there was some resistance to the work and that it wasn't going as smoothly or taking hold as much as I wanted it to. Perhaps the reading that someone would experience prolonged sorrow before the work was completed was true as well--but based upon my own intuition and my divinatory reading of the situation, it also meant that the work wasn't going exactly how I wanted and I needed to step things up and take a slightly different approach.
Watch the signs and see how the work you're doing plays out for you; then test those connections over time and your own process for interpreting such things will start to congeal. Personally, I think that is the best possible way to approach it, particularly given that there is no one right, universal mode of interpretation in this.
I hope that is somewhat helpful!