Thanks Tom and rocketman for the comments.
Tom - I've the same experience that you mention with the REPORT ERROR technique, nothing happens till you click SAVE. In fact if you use REPORT ERROR in a rule of the BO, it doesn't seem to make any difference whether 'dynamic' is ticked or not. The system behaviour appears the same.
On reflection, this probably isn't a big deal for the user, so bug or not I'm happy to use REPORT ERROR for now.
rocketman - Having read your post, I wonder if the 'dynamic re-calculation' was ever intended for 'lost focus/do process' style operation at all? The excerpt from the manual is obviously aimed at numeric re-calculation.
Maybe I'm barking up the wrong tree entirely here.
As usual, than you both for helpful posting.
Peter