To do this I always use a certain method -
There are the 3 grid modes in FPSC(changed by pressing B); generally people have it on the free-for-all style one where you can move and place entities freely. If you press B once, it's a little more strict on where you can place it, but not impossible to genuinely get it where you'd like it. Then pressing B twice would take you to the grid mode where it places the entity directly in the center of each FPSC grid square.
The middle grid mode, where you press B once from being in the free-for-all mode, is how I do it.
Being in that second grid mode, I place the candle stick, holder, whatever, this way it has a defined space on the tiny grid now. Then, pick my candle flame, and when I move it over the candle stick, by the fact that I'm on a limited grid mode, as long as I know I'm right over the candle stick, it will be exactly centered on it. (You can make sure you're over it first, before placing it, by using the Page Up and Page Down).
If you're not quite sure what I mean, I guess I could try and figure out a way to record my FPSC editor while I demonstrate.
But, assuming you're by default in the free-for-all grid mode for placing entities, just...
> Press B once
> Pick and place your candle stick where you want it
> Pick your candle flame, stay in that semi-limited grid mode (so don't press B again yet)
> Using Page Down, align the flame with the candle stick (easy now however because of the limited grid locations to place it)
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Here's an image demo to go along with it
I hope this helps!
Life would be much easier if I had the source code.