Formatting 'cluster text' labels/positioning

[wayfaringrob]

I don't use clusters all that often and I've discovered that the options for controlling the position of the 'cluster text' (labels) seem to be limited. You can select an existing style of yours as what it's calling a 'template,' but the positioning settings (like Offset X and Offset Y) are not available, nor are the traditional label positioning settings.

Yet my labels appear drawn at an offset rather than centered over the cluster.

When using one of the preset symbols, they center.

I'm stumped at how to get a custom symbol to center...am I missing something?