You won´t need any correction if you are focussing on the screen without any aid. When using the built-in magnifier however (which I would strongly advise for critical focussing), you will need it. I have -0,75 myself and changing the standard against a -1 magnifier made a real difference. I was able to focus with the standard magnifier as well, but now everything looks crisper, focussing became easier and faster for me. You could also buy a chimney finder, some of them have adjustable diopters.