Your heel should be at the very back of the clog (maybe even a slight bit over). In this way, you have the most secure fit and are best able to control your clog.
These stylish clogs offer protection for the back of your foot and keep the clog from slipping off. However, these are clogs (not shoes!) and will come UP & DOWN on your foot. Accordingly, you need enough room behind your heel to move freely. (If you rub against the back, you will get blisters!)