I find it interesting that the current average valuation of a YC company is 22.5 million which is almost identical to what it was 2 years ago when there was 210 companys "the average value of startups we've funded is about $22.4 million" http://ycombinator.com/nums.html
That is interesting. The reason it hasn't changed is convertible notes. Now most of the fundraising immediately after Demo Day is on notes. Since a note doesn't establish a valuation, we add zero to the total valuation for those companies. So the number you get by dividing the total valuation by 511 ends up being pretty conservative; it's equivalent to assuming all the companies that have raised money on notes will go out of business.
The average valuation of the 268 of those 511 companies that have valuations (by being acquired, raising an equity round, or going out of business, in which case it's zero) is $43 million.
Why not use the valuation cap on the note (in cases where there is one) in place of the valuation? Is there a reason you wouldn't consider it equivalent to a valuation for the purposes of a rough calculation like this?