Using BCC in other hosts there would be an overlay for the onscreen point picker and so that would act as a better visual reference for where the clone center was within the image. Unfortunately Vegas doesn't support overlays that way and I agree it's a bit awkward in this case since the XY point being adjusted is not obvious in the effect output. I just recorded a quick screencap movie with voiceover offering an admittedly unsatisfactory workaround and a suggestion for how we (BorisFX) might provide that functionality in Vegas by adding another choice in the "view" popup menu ;
http://www.borisfxresources.com/CreativeCowForum/BCCVegasPointControls/BCCV...
