You can make snapping points with the split polygon tool. So if you had 1 snap point, it would be at 50%. If you had 2 snap points, they would be at 33% and 66%. 3 would be 25%, 50%, and 75% and so on. By changing the snapping tolerance, you can set how "magnetic" these points are - if you set it all the way up, it will only snap to these points.