Split Polygon Tool, is not that bad. It has predictable result.

If sometimes you found out that the tool doesn't want to add another split, usually because it going from one edge to another edge is because of the topology, you are going from one face to another face that is not sharing the edge.

The percentage is measure percentage measure from the start point of an edge to your point along the edge you select. If you click on in the middle of a face, it will not show percentage but x y and z coordinate of the point.

Best regards,