How can I decide the error bound of my model?

Hello.
I have questions about the error bound of my simulaion results.
It is a fully hypothetical model and have no data to compare with. In that case, how can I decide the error bound of my model?
If it is relevant to the numerical scheme, how can I obtain the error ranges when using the default setting of FEFLOW?
Please, I need you guys' help. It would be very greatful to recommend any refernces on it. 

My understanding from the white papers is that error tolerance is related to the maximum head elevation.
For instance, If your max head elevations are 2 m (a.s.l.), and you specify a error tolerance of 0.001, then the change in the norm you are using is 0.001 x 2 m = 0,002 m.
This changes if your max head elevations are 2000 m.a.s.l  (0.001 x 2000 = 2 m)

You're right, Jegs.

Thanks. It's pretty useful advice and I'll check the manual!