Optimal Alphabet Stepping
Charcoal, 41 bytes

≔ＥＥβＥθ∧μ⌕⭆β§β⁺⌕β§θ⊖μ×κξλ⎇⊕⌊ιΣι⌊ιθＩ⌕θ⌊Φθ⊕ι


Try it online! Link is to verbose version of code. 0-indexed. Explanation:

Ｅβ


Loop over the 26 step sizes. (Actually I loop over the lowercase alphabet here and use the index variable.)

Ｅθ∧μ


Loop over each character of the input after the first.

⭆β§β⁺⌕β§θ⊖μ×κξ


Loop 26 times and generate the string of characters resulting by taking 26 steps at the given step size starting (0-indexed) with the previous character of the input.

⌕...λ


Find the position of the current character of the input in that string, or -1 if not found.

Ｅ...⎇⊕⌊ιΣι⌊ι


Take the sum of all the positions, unless one was not found, in which case use -1.

≔...θ


Save the sums.

⌊Φθ⊕ι


Find the minimum non-negative sum.

Ｉ⌕θ...


Find the first step size with that sum and output it.