One of the issues when shooting mitres, such as those for narrow picture frames or mouldings is that the end of the mitre has no support. By contrast, shooting mitres, such as wide boxes sides, which is across the grain, there is support at the edge (although it is weak and apt to spelch).

In all cases, it pays to chamfer the end of the mitre before making the cut. You do this with square edges (drawer sides and box sides) to avoid spelching; you need to do it with frame and moulding mitres as well. In the case of the latter, it is to add support to the edge, or it will just fold over in the cut, and leave a ragged edge.

A second factor is ensuring that the board does not move. Clamping boards for mitre cuts is important.

The Rule Trick is unlikely to be the cause of a miss cut.

Regards from Perth

Derek