I have found veins left and right (literally going down to bedrock ore was on the left and right).

Last night I was making a pit of death (don't ask) and in that process I found ore 25 blocks from bedrock. Iron Lead and Coal.

I have yet to find Tung.

I broke off a path from my stairs at about 25 from bedrock and found more.

So at the very least on average I would say around 25 blocks from bedrock is a good place to look. The reason I say form bedrock is people settle/start at different areas of sea level. If you are high on a hill or low in a valley counting down would not be as accurate.

Your mileage may vary.