Merlin, there is a indexing fix screw on the Dlask just like the RA adapter. However, ideally, you want the whole end of the UBR stock to butt up against a surface... like the end of an AR receiver and end plate, as the UBR was designed for. What you're showing is certainly serviceable, but not ideal. The only thing keeping the tube and stock indexed is that little fix screw. That's not a lot of support. Not as much as there would be if the indexing lug on the end of the stock was set in a recess at the end of the receiver/adapter. If you ever need to pogo the gun or bash zombies with your stock, you could easily knock that UBR out of index and cause the fix screw to damage your tube.
This is why I'd recommend against UBR unless you have a NEA adapter. Otherwise, a CTR or other Magpul stocks w/o integrated tube would be better. With those you'd at least have the castle nut to help you maintain index and provide support/strength.
Through Bing image search, below is a picture someone here posted. It's a UBR installed onto a Dlask adapter. Again, the only thing holding index is that little fix screw. You can almost see the exposed lug, just hanging below the bottom of the adapter. It has nothing to interlock with.
Here is another picture someone else posted here of the NEA adapter with UBR. As you can see, the NEA adapter emulates the end of an AR and mates up to the UBR fully. You may have to jack up your monitor's brightness to see it better. Camera should've been focused on the gun for correct exposure. This adapter shifts the axis of the tube higher in relation to the barrel to make room, so the lug at the end of the stock can mate up to the adapter. IIRC, the adapter itself is also ever-so-slighlty taller than the end of the XCR and you can kind of see that in this picture.