Great. I am on ST 6 as well.
7 days time makes a thread on this buzzing forum a really old topic but I could not simply let this get off my mind.
After playing more with the middle-mouse rotate, I noticed it is not necessary to first middle-click to get the red/pink dot to rotate about nor pick or specify it on model geometry and then proceed to actually rotate.
This can all happen in one 'Go'.
Might already be known to many and also somewhere deep in the help files but what I found was I could keep the middle-button pressed a 'bit longer' and Solid Edge automatically 'grabs' the pink dot following which I could continue to rotate the model.
So all I did was without starting any command and just by making sure that the mouse cursor 'is over some geometry' like a face, edge or a vertex and 'not in empty space', pressed the middle-mouse-button and kept it pressed for a bit while to create and grabs the red dot in one shot and continued to rotate the model.
Without any geometry under the cursor, the model would rotate unpredictably.
Not sure if this solved your problem or if it is already solved but give it a try and update here if possible.
Below is a video: no geometry under cursor means un-predicatable rotatation.
And with geometry under cursor, the red-dot appears automatically, just keep the middle-button pressed a 'little longer'.