To the best of my knowledge, there is no "right way" to tune the seventh string. I know several rockers that tune to a low A and keep it "legit" on the other 6. Some others "baritone it" and tune it all down starting with a low D. I saw low D (then legit on up)as "a standard" at Kerrville on two seven strings. And another down there tuned to a D9/6 or (depending on the day) a C9.

I might try something totally different, say an E6.