I've always primed my beer at the same rate (5g corn sugar/L) but a recent spate of gushers has made me think that perhaps this is too much - is 4g/L more sensible? or even 3g?
4 g/L is a reasonable amount of priming sugar. It will add around 1 volume of CO2. Beer fermented at room temperature will contain round 1 volume of dissolved CO2, so adding the priming sugar brings the total CO2 volumes to 2, which is a typical carbonation for many styles.
There are a lot of priming calculators available online. You enter the beer's temperature, volume and desired carbonation level. The calculator will tell you how much sugar to add. This one comes with a nice explanation of how the calculations work.
If you're getting gushers, there are other possible explanations.