When you get the position of an axes, the first and second index are the start location of your axes in the x and y dimensions, respectively. The third and fourth index is the width and height of the axes. Try typing this into just the command window, in turn:
It will probably give you something like
0.1300 0.1100 0.7750 0.8150
(this is for my screen). It means that the x axis starts 0,13 of the way in, from the very leftmost part of the figure, and it then proceeds a further 0,775 whereupon it stops.
So what you want is for the x location of your textbox to be your axes start point, p(1), plus your axes width, p(3), minus the width of your text box. How to get that width? Well you have your ann handle, so ann.Position gives you the width (and height) - again in the 3rd and 4th index.
On my screen resolution this gives a textbox in the top right.
(mind you I hardset axes limits and the string, since I obviously dont have the variables you do :)
dim = [(p(1)+p(3)-w) (p(2)+p(4)-h) w h];
str = ['Max dist (+/-0.1mm) = 0.7203'];