I see you are cruising on the Wonder in April. On my cruise on the Wonder last April I think the first dinner seating was at 5:45pm, so if your appointment ends at 6:15pm you would be 30 minutes late. But eating at Beachblanket (the Wonder's version of Cabanas) is really a nice alternative. We ate there one evening and really liked it! We had a table at the window and enjoyed the view of the sunset over the sea. The food was delicious, it is more or less the same as in the MDR, kind of a selection of the various menus being offered. Overall we liked it so much that we decided to do it again during our next cruise. Personally, I would not want to have to rush out after my spa appointment to get quickly dressed to arrive late for my seating in the MDR. That would be too much stress and I would fear that it would take away from the wonderful relaxation the spa is supposed to bring with it.
Alternatively: Is there a chance that you can change the time of your spa appointment? It might be worth it to enquire about this once you board the ship!