Why don't you have a look at the already available vectors with His-tag and enterokinase sites and use those. When cleaving the tag off, it shouldn't be important if you add one or more amino acids (not bases, but base triplets!) between the tag and the EK site. The cleavage occurs after the site, so it is more relevant to think about the accessibility of the cleavage site that could depend on your downstream target protein. If you don't want to have any additional amino acids at the N-terminus, you will have to accept the risk of incomplete cleavage but this will most likely be not depend on the number of amino acids before the cleavage site.