Setting A Benchmark in GPS Application

I am a newbie playing with GPS Applications
I have this GPS Appliacation and I would like to set a bench mark to reference a local coordinate system. for example: if a UTM coordinate is
E-445000,N-460456 bench to E-0,N-0

The idea is when i am over a reference point i click a bench button and then the you take the long utm cooridante and reference a smaller predetermined number.

also does anyone know how to select an entered value, for example a gps height offset, i would like to enter an offset height.

Any help would be great

Thanks
Joey