[Qgis-developer] scaling & mupp of canvas

[nitin quick]

greetings
For conversion of user units to canvas QgsMapCanvas class provides
mupp(), getscale() .
i want to convert 10km distant in equivalent of that. What i am doing is

10000 * mapUnits()
-----------------------------
mupp()* getScale()


Is it correct since ouput donot look like what was expected.
Also i am not able to undestand what scaleBar plugin denoted there. It
has a combo box letting user to specify size 30m/km etc. Wjat it is
exactly trying to convey .I went through code also but confused.

regards
quickNitin