[racket] display problem

Hello,

I have a struct which contains text and the place of a cursor.
The place are 2 strings which contains the character on the left and on 
the right of the cursor.

Now I have to display this.
But I wonder how I can set the cursor on the right place when I can't 
know the numbers where between the cursor is placed.
I know that the characters are 11 pixels.

Anyone tips ?

Roelof