Any tips for checking boot size?

**embarassing story follows**

I've recently wondered if my ski boots are too big or not. For years I have
been buying shoes too big, I measure a 9 now and had been buying size 10/11
shoes for years. My boots are a 9.5.

I did the shell test and seem to remember a fair gap behind my heel when
the toes were touching the front of the shell.

Is there any other tests I can do?

regards,

greg