There are heaps of different styles of lettering used in cross stitch. A lot of lettering charts are free but you can also buy cross stitch books with lots of different lettering styles.
If you want to centre a word(s) you will need to count the holes used for the widest part of each letter and the amount of space you want between letters and words. Halve this and count from the middle of where you want the stitches.
Most charts suggest 2 to 3 spaces between letters and 5 between words. Unfortunately this does not allow for "kerning". Kerning is something computers do automatically (typesetters also use it). It automatically spaces the letters so that they appear evenly spaced even though they are not.
For example my son's name Troy would look strange if the lower case r was two spaces away from the widest part of the capital T.
To remedy this make a copy of your letter and cut and paste them onto a piece of paper. Use that length for the total.