Setting the wick
Updated
A wick a few millimeters off center burns one wall hotter than the other for the whole life of the candle.
Centerd and vertical
Centerd, so heat reaches the wall evenly. Vertical, so it stays centerd as the candle burns down.
An off-center wick gives an uneven melt pool, a wall of unmelted wax on one side and a hot spot on the glass on the other. It is a leading cause of both tunneling complaints and vessel failures.
How to hold it
A wick sticker or a dab of hot glue at the base, pressed firmly onto clean dry glass. A sticker on a dusty base lets go during the pour and the wick floats up.
A wick bar, a clothes peg or a purpose-made centring tool across the top to hold it vertical while the wax sets.
Check after pouring and again a few minutes later, because a wick can lean as the wax moves.
Priming
Many wicks come pre-waxed. Where they are not, dipping in melted wax and letting it set stiffens the wick and makes it stand straight.
A primed wick also lights more readily on the first burn, which is the burn the customer judges the candle on.
Two wicks
Placement becomes its own variable. Too close and the flames merge into one hot column over the center; too far apart and the middle never melts through.
Both distances are found by testing rather than by geometry, and it is worth marking the positions on a template so every candle in the batch is wicked identically.