The problem is that blank_str = str_create("#length"); creates a string of only 7 characters long. #, l, e, n, g, t, h = 7. The "#num" syntax only works with string initialization, and only with literals.
You could try: var length = str_len(my_str); length = 175 - length; int i = 0; for(i = 0; i < length; i++) {str_cat(my_str, " ");}