He was about to reach for the bag when she showed him her arm. He couldn't help but notice her reaction, either. Ah. He reached for her arm with one hand at the same time as he reached for a tape measure that was sitting on the worktop amongst a pile of other tools.
"So, this'd be your... third hand? If you want, I can make a couple of them so you'd always have a spare." Maybe if he kept talking, it'd help distract her from what he was doing. Fuck knows, he'd hated it when he'd first needed measured for a new leg. Of course, he was talking about her hand... well done, genius.