From the Compendium : When you cast this spell using a spell slot 3rd level of or higher, the
damage increases by 1d8 for every two slot levels above the 2nd . This means that Spiritual Weapon does an additional 1d8 damage when cast using a 4th-level spell slot, an extra 2d8 when cast using a 6th-level spell slot, or an additional 3d8 damage when cast using an 8th-level spell slot. The (0.49 * Spell Level)d8 compensates for this mechanic. I tested the spell just now, and it functions accordingly.