Such wilt thou be to me, who must, Like th' other foot, obliquely run; Thy firmness makes my circle just, And makes me end where I begun.
The end of the poem spells out the metaphor and winds down the poem with more praise for his wife. Line thirty-three connects the fixed foot firmly with his wife.
This stanza is similar to what is called the 'turn' in a sonnet (Donne wrote lots and lots of those). Everything before the turn is metaphorical and convoluted, but now at the end he makes everything plain.
Once again in line 35 Donne praises his wife for her faithfulness, for sticking with him even as he runs all over the place. We can connect the word "firmness" with "fix'd."
Of course Donne means that the center foot makes a circle accurate and perfectly round, but "just" also carries with it a legal or even moral connotation. It's possible that Donne is saying that the faithfulness of his wife will keep him from straying while he is away. (Let's hope that she didn't need that kind of reassurance.)
The last line has a nice ring of finality to it. We've really come full circle (see what we did there?). Seriously though, this line is Donne's final promise, his final reason why they shouldn't mourn at his parting: if they are both firm and strong, he will be back soon enough—right where he belongs.