the chain ring shouldn't matter but since we're talking about the small cog, I'd use the big chain ring.

I doubt there's sufficient range on the tension adjust for this. I'd unclamp the cable at the rear derailleur, and pull on it while clicking the 'up shift' button or whatever, to see if it clicks to one more higher (smaller) gear position. if so, I'd screw the tension adjuster most of the way back in (guessing its been cranked out moderately far), pull the cable snug, and tighten the cable clamp, then go through setting the index adjustment on a middle gear, and then try for low gear again.