To calculate the level of light needed for your room, find your the size of your room in square meters and multiply by 250.

If your room is 4m x 5m, the area would equal 20 square meters. Multiply this by 25, giving a wattage of 500 watts, or a lumen level of 5000 lumens. 

This is down to personal preference, if you like the look of a dimmed bulb then this calculation won't work for you. 

If you're unsure, it may be worth fitting a dimmer switch so you can control the level of light manually, but it' important that you make sure that the bulb that you're choosing is a dimmer bulb.