Is your pen inaccurate? Do you have lots of time? Try this calibration!
This really helped me out, so I figured I'd share it.
This is the script to run a 273-point pen input calibration, which'll probably make your pen input more accurate (including on those damn corners); it really improved my Surface Pro's pen input. Align your pen's tip, not the on-screen reticule, with the touch target, and tap on through.
Hope this helps.