how to find the central character of a word?

i am working on an algorithm in which i have to select a keyword from a text document and then find the central character of that keyword. for example if 'this' is the keyword then how can i find the central character of 'this' word?

[Yao Li]

What's the central character of 'this'? The word follows 'this' or the the cetral letter of 'this'?

[Walter Roberson]

If the text is proportionally spaced, do you want the letter whose center is closest to the centroid of the representation of the word, or do you want to determine the individual letters and then find the letter that is half-way through the list of letters?