My recent completed quilt is 45x65 and I spray basted the entire thing. I used 505. I also basted it on my bed for the first time. I lay my backing down, smooth it out, then batting and smooth. I think pull back the batting and spray then do the other half. I then usually have to go back and do another application on the borders. The spray helpd throughout quilting and my kidlet wrapping up in it before it was finished. I don't think I'll ever pin again.
As to the original question - I do stabilizing stitches around the center, ususally, and then I just go from there. Definitely try the spray again. It is a wonder!