This is a great site for making routes, but I think you ought to match the calculation of distances better. If I wish to make a route of 100 km in length, I get routes generated as short as 41.5 km. That is not really usable, and I have to continue making routes, until I get one that is close to the 100 km. It think you need to make your calculations for this better, perhaps within +-10% of what we have entered. More than 50% off is just not close enough.
Thanks for the comment. The calculations vary in accuracy depending on the road density in a given area. If there are not that many roads, the algorithm can struggle to get close to what you need. As you keep trying, though, it learns that and tries to compensate.
If you have this problem too much, though, you can always start with the result returned and then modify the route by hand to get you closer to the length you want.