Why a three-digit number? 108 and 111 are good numbers to use, but for different purposes. It solely depends on the nature of your workings. Do you wish to detach/remove unhealthy habits or increase/improve something? Using a high number of repetitions daily for 40 days takes much discipline and, therefore, may not be the best choice for newbies or the less experienced.
The numbers 10, 18, 20, 36, 40, 54, and 80 are also good numbers to use depending on the working. Workings are based on an accumulation effect and, therefore, still hold great power when done daily for a 40-day cycle, and even greater power if done for 80 or 90 days. Again, it takes discipline and consistency. Though you are the best judge and know your capabilities here. I am just pointing a few things out.