What distance-to-spot ratio do I need?

A higher ratio (e.g. 12:1, 16:1, 30:1) lets you measure a smaller target area from farther away. Cooking and home use is fine at 10:1 to 12:1; high-temp industrial tasks like kilns or forging benefit from 30:1 or higher so you can stay back from the heat. Remember the measured spot grows as you back away.