pos=Geo.Position; % save as array
x=pos(1)+pos(3)/2; % midpoint position horizontal
y=pos(2)+pos(4)/2; % midpoint position vertical
text(x,y,(['mean = ']));
NB: however, that text() uses the axes positions, not the figure position coordinates that are retuned by the 'Position' property and that are relative to the actual monitor/display, NOT the axes/figure itself. Those two coordinate positions don't relate to each other.
If the idea is to put the text at the middle of the x- and y- current axes, then using
hTxt=text(0.5,0.5,'Mid-Axes Text','units','normalized','horizontalalign','center');
will put the text at the middle axes range irrespective of x-, y- data limits without trying to compute pixel locations.
