One Time Pad OTP : Optimal modular base per given plaintext space? Background:  my question at : 

My pencil and paper One Time Pad works fine without modular math ...... or does it?

I am trying to understand, from a layman's point of view, how to make the best one time pad scheme.

From responses to my above referenced question in another post, I understand now why I should use modular math in the scheme, and that the keyspace should be at least as large as the plaintext space.

So, moving beyond those points, I would like to try to understand the best modular base per given plaintext space.   For example:

My alphabet's plaintext space is 40 characters, numbered 0 through 39.

So, I am using mod40 math.   But I am only using mod40 math because I read elsewhere that my modular base should equal the size of my plaintext space.
But I also know that my OTP will still function if my mod base is less than the size of my plaintext space or more than that size.  So, in my example, I could use mod30 math or mod50 math just as well as mod40 math.

Therefore, my question is: what is the importance of the mod base in relation to the plaintext space, and, is there an optimal relationship between the two, and, if so, how can I know the optimal ratio of  "mod base/plaintext space" in any OTP system?
I am assuming that your scheme encrypts $x$ to $x + k \bmod N$, where $k$ is the key and $N$ is the modulus.

If your modulus is less than the alphabet size, decryption does not work.

Suppose your alphabet size is $n$ and you work modulo $n-1$; then the plaintexts $0$ and $n-1$ are considered the same, since $0 \equiv n-1 \pmod{n-1}$. Thus, no matter your key, they will be encrypted in the same way, and when you try to decrypt, you cannot tell them apart.

If your modulus is greater than the alphabet size, you do not have perfect secrecy (hence not an OTP).

This was already covered in answers to the referenced question.