Rancidtom is spot on.
There are no magic numbers as we are all different - up to a point, also second taking a target with you and making sure it fuzzes out a bit and the front sight sharp and rear sight a bit fuzzy, opticians can take an experienced guess at the numbers but you need to confirm it by waggling the pistol at the end of your arm and using relaxed eye muscles to view the picture. All my lenses over the last four years have been so, so until I actually confirmed it by using the pistol. Also make sure that if you use dedicated shooting frames for the new lens make sure the thing is adjusted to place the new lens the normal distance from the eye like ordinary glasses.