I've seen plenty of builds have the booster overhang the shell and I believe some of the hero packs also had this feature, but correct me if I'm wrong, maybe half a cm is acceptable but surely a cm like in this picture is too much overhang?
Apologies if this doesn't make sense, I'm not exactly a skilled builder and I'm learning as I go along really which isn't easy for someone who has never been artistically blessed!!
I know this image isn't the best but it was the best because I had the booster heavily taped in place and propped against a load of pillows just so I could snap this pic!!