[shawndrost]

Maybe the best solution would be to show a scalable picture of some coins, and ask them to hold a coin up to the screen make the image output the real size.

[zach]

I thought of that, but it doesn't internationalize well. A USB plug is probably more universal but a little weird.

[asmosoinio]

For me screen size worked fine. This would be a lot slower than the current functionality, right?

Maybe have such a system as a back up, for fine tuning. Credit card would work for me, I always have some odd card in my wallet with me.