To calibrate the touch screen of the nüvi 765T, see the instructions on page 51 of the [Only registered and activated users can see links. ] (click link).
Just tried it on my own nüvi 765T. It works, but in step 3 you have to move the power switch to the "locked" position before the splash screen appears. So while the screen is still black! If you get a message telling you the screen is locked, you've been too slow.