In some science and engineering literature, it is common to right the ordinal form of an unknown number m as mth. While the ordinal form of number 1 is first, it is also commonly written as 1st. I was wondering what would be the most appropriate way to write the ordinal form of (m-1)? (m-1)th or (m-1)st?
(m-1)th is still appropriate. The expression in parentheses will still be read without the ordinal, so consider how it sounds when spoken—em-minus-oneth makes more sense than em-minus-onest.
I should also note that it's common to avoid the problem entirely through rephrasing (e.g. 'the value at index (m-1)'), but that may not be applicable to your field.