Get the OOXii Near Vision Chart and ask the person to hold it in good light at a comfortable reading distance. This is usually about 40cm. Ask the person to look at the smallest letters they can see comfortably.

Hold the +1.0 paddle in front of the person's eyes, then the +1.5 paddle, then the +2.0 paddle and so on, and ask the person which they prefer. This will be the paddle that makes the letters clear at a comfortable distance without any eye strain.