seems simple but i can't make this thing work 
this is form Robin Hills HeightTool.rb
def onLButtonUp(flags, x, y, view) #called when left mouse button released
txt='Height: '+@pt[2].to_s
model=Sketchup.active_model
e=model.entities
t=e.add_text txt,@pt,@vec
end
what i want to do is make this tool work relative to a set zero level... not the files axis zero. i get the user set level with the same method as in the drop ruby script so now i have a @@level=The user set value
but performing
txt='Height: '+(@pt[2]-@@level).to_s
doesn't show the right value
@@level.to_s alone works well but the subtraction doesn't
what am i doing wrong here 