The first thing to calculate is the size of the room, including the ceiling height. The other important factor is the application in which the lights are being installed, or simply what level of light is desired within the space. You may want softer light in bedrooms, but brighter light in the kitchen. A general rule of thumb for most recessed down lights ranging from 700-1000 lumens would be to position them approximately 1-2 metres apart. For example, in a simple room that is 3x3m you may choose to use four D700+ downlights, whereas in a room that is 4x4m you may opt to use of four D900+ downlights which offer a higher light output for the larger space.
Each one of our products outputs a certain number of lumens which will illuminate your space to a certain level. You can find an interactive light level calculator towards the bottom of each product page. It will allow you to input your ceiling height and then demonstrate what sort of beam spread and light output will be achieved. Ideally you would have some overlap in the beam spread between each downlight.
You may also want to factor in the specific application for the lighting. Perhaps you're installing recessed downlights for general illumination of the room, but you also want to use some track lighting to highlight specific features in the room.
If you have a specific plan you can contact us for a free lighting design.