The Golden Ratio
Here are some ways:
Amount of furniture in a room
Proportion/sizing of furniture in relation to other pieces
A coffee table should be about 60% of the size of the sofa it serves. On that coffee table (because a table is primarily supposed to be clear) it should be about 40% decorated with items and 60% open.
If a sofa takes up 60% of the seating in a room, the accent chairs should make up the remaining 40%.
In a dining room, 60% of that space should be furnished with table and chairs, right? So within that furnished 60%, 60% should be taken up by table and 40% by chairs! Starting to see how this works?
Bed/headboard: Bed 60% / Headboard 40%
Color in a room
If you are doing 2 colors, the dominant color in a room should take up 60% of the painted area. The secondary color would take up 40%.
If you want to do 3 colors, it's 60/30/10 and the 10 is usually a decorative accent and not not. You can see it here: 60 beige, 30 teal, 10 red