[GRASSLIST:76] interpolated values out of range

[Stefan Istvan]

Hello List!

I have some vector points with a value attached to them, and I would
like to interpolate a raster surface from them. 
My problem is that the resulting raster map contains values that are
smaller then the minimum value of my vector points.
I set the smooth paramter to 0, in order to the resulting surface cross
exactly the vector points.
I also set the tension paramter to 180 because during the interpolation
grass said that overshoot occures and suggested to increase tension.
The imported vector point values range from 10-35900, but on the
resulting raster map there are values far below zero (-2000).
I also tried so set smooth to default (0.1), but still get values below
0.
Is there any method to calculate the value of tension and smooth, or
just trying?

Thank's for the help in advance,
Istvan