Though BUG (IES TM-15-11, Addendum A) is quite hard to do, it is easy to understand, it gives you numbers for the "amount of light", wasted in various non-useful directions.
Light issuing from High to Very High (H and VH) zones can cause glare from a distance and is anyway wasted light. These zones, both foward and back are used for the G rating. These are the red areas in the schematic below:
The blue areas in the image above (Upper Low and Upper High) show the zones where light is wasted skywards, allowing jet passengers to have a pretty view of the city below, but not much good to road users.
It is best to have most light in the Foward Low and Forward Medium zones, which shine light onto the road surface relatively close to the luminaire. These areas are green in the diagram above. FL and FM zones are not used in the calculation, they are not zones of wasted light.
So with the idea that it is a measure of waste, high numbers are considered bad, and low numbers are considered good:
A good mnemonic would be: "It BUGs me all this waste!".
Here is another example :
In the above example 13.7% of the light is wasted in the Uplight, hence the U5 rating. The B2 and G2 ratings are not brilliant either.
When evaluating a BUG rating the BL and UL zones are not used, because these are considered to be useful light which shines down onto the road or sidewalk. The BL and UL zones in the above example are at the bottom of the circular diagram, from 30° to 30°.
Compare the above rating to this one:
Notice how no light is wasted into the sky, none is wasted shining light far away (the zones between 60° and 90° back and front). As a result the BUG rating is much better, B1 U1 G0.