What is the chance of iron spawning at a particular level?
Solution 1:
From the Minecraft Wiki article on iron ore:
Iron occurs only around from bedrock to slightly above sea level (layers 1-63). However, almost all of the iron ore will be at the lower levels. On average, there are about 77 iron ores per chunk.
So, you will find the most iron ore if you look pretty close to bedrock. You can see what level you're on by pressing F3 and looking at the 'Y' coordinate.