More like 3.25 cm, in general -- it really depends on the laxity of your scalp.
Looking at "beauty" charts, which attempt to create mathematical formulas for female beauty, the there are "ideal" ratios for the proportions of the face. Looking at a head-on profile, divide the face into three sections -- bottom of the chin to middle of the mouth, mouth to center of the eyes, and eyes to the hairline. The distance from the mouth to the eyes is more or less fixed -- the procedures available to us today generally can't change this. This is the distance to compare the eye/hairline distance, which is "ideally" the same. (Mouth to chin is about 62% of the center distance, "ideally.")
Now, there is are a couple good reasons for doing a scalp advancement (as a part of FFS) even if your biggest problem is recession at the temples. Namely, it still reduces the area to fill in with transplants. The fewer transplants needed, the less cost for the procedure, in general. As well as less donor material from the back of your head. Secondly, if your hairline needs an advancement just to get close to that ratio, it's probably going to be beneficial.
Conversely, though, if your hairline is already close to ideal, you might not want to have the advancement. You can just fill in the corners and have a "high forehead," as it were. This allows for greater strip harvesting at the back of your head, because you'll have great laxity there without the advancement, as opposed to Follicular Unit Extraction, which is both more expensive and requires the donor area to be shaved. On the other, other hand, FUE doesn't leave a scar on the back of your head.
So it's really not a cut and dried answer -- it ultimately depends on your unique individual characteristics as to which route is the best for you.