We're not magicians!
There could be many reasons to have wrong result and when I say wrong we could also have a discussion on "how wrong it is"...
Some of them will comes from the model itself (geographical projection, vertical datum, ...), other could comes from the tuning parameter (friction, turbulence ...)
As indicate in the forum rules: give a clear description of your problem with all required files will probably help people to give you appropriate ideas.
If you really need a precise answer here, I would say
42
Regards