--- this has been edited as I realised the question was silly. replaced with helpful information ---
I want to confirm the following:
Given ciphertext "AGHTDIX... [cont.]" (C)
and crib (a word which I know exists in the plaintext) "WITH" (P)
I am attempting to find the key (K) by shifting "WITH" one spot down the text each time like so:
A G H T D I X W I T H . . . . . . . . . . . . W I T H . . . . . . . .
And calculating it using the formula (multiply the ciphertext matrix by the modular inverse of the plaintext/crib - sorry, I don't know how to insert proper formulas here):
[K] = [C] x [P]-1 (mod 26)
My question -- was -- really silly as I forgot that the Hill cipher encrypts in pairs. Crib dragging should then adapt to include letters which could come before or after the known plaintext.
A G H T D I X W I T H . . . [? W] [I T] [H ?] . . . W I T H . . . . . . . .
Answer below explains in better detail. Thank you :)