EN13537 requires a thermal mannequin test which produces four temperature results — upper limit, comfort, lower limit and extreme.
These interpret as:
For the purpose of these measurements, a "standard man" is assumed to be 25 years old, with a height of 1.73 m and a weight of 73kg; a "standard woman" is assumed to be 25 years old, with a height of 1.60 m and a weight of 60kg.
Of these four ratings the most useful are the 'Comfort' rating and the 'Lower Limit' rating. Both these ratings assume the sleeper wears a baselayer, hat, socks and insulating pad. In theory Men should be interested in the Lower Comfort Limit rating, and Women the Comfort rating. Therefore, if you are a man and the lowest temperature you expect to use the bag in is 0C then bags with a 0C Comfort Limit will be your starting point - then factor in other considerations that apply - see below:
People with differing metabolisms will feel the cold more than others, some of us will 'sleep warm' and others will 'sleep cold' - a knowledge of our own tendency will also inform our decision. If you are a man and the lowest temperature you expect to use the bag in is 0C then bags with a 0C Comfort Limit are the starting point but if you are a 'cold sleeper' you may want to go for a -2C or even -5C bag.
We may expect temperatures of -5C in the area we are going to - but if we are sleeping in a two skin tent the air inside the inner tent will be up to 5C higher than the outside air temperature. We would therefore not need such a low rated bag compared to if we were sleeping in the open.
A sleeping pad provides insulation from the ground as well as comfort - if you are using a pad with a high insulating value then this will reduce the rating of sleeping bag you will need. Remember the standard test assumes an insulating pad is used - so its only if your pad is exceptionally warm should you adjust your EN rating.
A sleeping bag liner is useful to keep the bag clean - however it will also add a few degrees to the rating of a standard bag. You can also buy specific thermal liners to enhance the performance of your bag by as much as ten degrees.
It may be that you will use your sleeping bag in fairly warm temperatures for most of the time but occasionally will want to use it in winter (say) when temperatures will be somewhat colder. In this case you could wear more clothing - sleeping in thermal leggings and down jackets/shirts can really enhance a sleeping bags performance, and whilst it may be less comfortable than sleeping in just a baselayer - it may be just for a few nights and therefore preferable to buying a much heavier bag that may be too warm most of the time.