the Texture command (from gSPTexture) sets the tile descriptor # to use as Texel 0, and then that # plus 1 is Texel 1 (unless mipmapping i think, but i haven't handled this yet) Is this what you're asking for?
Hmm... I always though RENDERTILE (0) on settile was ALWAYS T0, and T1 was 1. Furthur more, MIP-MAPPING uses TILES 0-6 starting with the value from gSPTexture. That is just my understanding. <enter disclaimer here>