Scandinavian-Style Single Spey
See if this helps when you are trying for a big angle change on a Scandinavian-style Single Spey:
1). Lift the line at the dangle. The goal is to clear most, if not all, of the shooting head from the water.
2). Pivot with the hips first and keep your arms locked as you do so. Keep your right foot forward from the left bank as it prevents over-rotation.
3). Throw the rear loop/Spey back cast with your arms as late as you can.
4). Make the cast.
Ultimately, you want to blend Steps #2 and #3 together. It is deceiving when you watch a very good Speycaster: it looks like they are doing most of the work with their arms, when, in fact, their hips initiate the turn with a Single Spey and the arms are reserved for the actual Spey backcast.
As always, it is best to work towards a large angle change with a Single Spey in small slices; don't try to go for it all at once. Incremental gain is good.