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.
- the beer had not finished fermentation prior to bottling. Make sure the specific gravity is constant over a period of a few days.
- the beer had more dissolved CO2 than typical. Probably due to low temperatures
- bacterial contamination. Some bug will consume sugar that brewing yeast leaves behind, producing excess carbon dioxide.
- Incomplete mixing of priming sugar prior to bottling. Some bottles will get more priming sugar than others, and will gush. This question has answers pertaining to mixing the priming sugar completely.