How much wax you coat on the wick beforehand has little or nothing to do with the strength. When you pour the candle, the wax mixes. If you want to be able to change wicks, a wick pin is your best bet. Unless the size/shape of the mold changes at the top or bottom of the mold, the height should not have anything to to with how it burns. You didn't mention what shape it is, just the dimensions. What wax and FO did you use? Is it possible you trimmed the wick too short before lighting? How does the melt pool look? Added: Keeping in mind, I use wick pins. However, when I make a tester, I leave an extra inch of wick hanging out on bottom, folded over. If I need to change, I use needlenose pliers. I grip the end with the tip of the pliers then wrapthe wick around the end. It makes it much less likely to shred the wick. If it isn't going to come out, it'll break off at the candle base.