That's a nice hack, except that it now takes away the information about the total number of pages. While having a percentage estimate is nice, I think people would still like to know which page out of how many total they are on. That is how we are all used to reading our books. Maybe have it as 354/400 (56%). That would perhaps be more useful.
this.setVariable("BOOK_INDEX_COUNT", i + " / " + c + " ( " + per.toFixed(1) + "%"+ ") ");