I consider 4x Krosaan Grip, 4x Chalice of the Void, 3x Pithing Needle and 1x Tormod's Crypt/Bojuka Bog as fixed sb slots. This leaves me with 3 meta-slots which will usually be either 2x Oblivion Stone, 1x Smokestack or 2-3x Null Rod and 0-1x Smokestack/Oblivion Stone. Only 2 meta slots seems to narrow especially considering that at least 1 of my sb cards (Crucible) would be sided in almost by default therefor not actually beind dedicated to certain matchups.

By the way, Null Rod is really strong; hurting Countertop, Fast-artifact mana for combo, Crypt and Relic as well als destroying Affinity. Played against Affinity three times in my last tournaments and it wasn't even funny...for them ;-)