Based on your scheme, we could probably say that items within about 1 - 1.5 points of each other are a side-grade...though that expands a little with higher point items like chest and shield.
So shields, for instance, would look something like this in the BC format (truncated a bit):
1- 126.42 [Hero's Surrender] - (H) Naxxramas, Patchwerk
1- 126.42 [Wall of Terror] - (H) Naxxramas, Kel'Thuzad
3- 124.16 [Barricade of Eternity] - The Eye of Eternity, Malygos
4- 120.29 [Crygil's Discarded Plate Panel] - Emblem of Heroism, 35
4- 118.97 [Titansteel Shield Wall] - BoE Crafted, Blacksmithing
4- 118.06 [Royal Crest of Lordaeron] - (H) The Culling of Stratholme, Mal'ganis
4- 117.31 [The Skull of Ruin] - Naxxramas, Grobbulus
8- 111.26 [Bulwark of the Noble Protector] - (H) The Nexus, Keristrasza
9- 107.08 [Leeka's Shield] - The Culling of Stratholme, Mal'ganis
10- 101.33 [Saronite Bulwark] - BoE Crafted, Blacksmithing
10- 100.17 [Riot Shield] - The Violet Hold, Xevozz