479......I'd be interested to hear how or why he is too optimistic.

Whenever somebody comes up with a number which might seem optimistic, for me it is primarily a question of "optimistic about what?"

Is the number intended to be what a share's actual fair value is? I wouldn't have a problem with the range of his number.
Is the number intended to be what he thinks it will be trading at in the markets, some time not long from now? I don't buy it.
Is the number a "sum of parts" valuation of what the firm would be worth if broken up and all parts trading at valuations comparable to others in their industries?

The last one would be a bit of a waste of time. First, the firm isn't being broken up, and second, market prices are not really a useful yardstick of value.

For me the big thing is that actual fair value and likely market price are pretty far apart. A share of Berkshire is usually somewhat cheaper than what it's actually worth.

Jim