The geometric mean is a reasonable simple solution to the problem.
If you want to be really analytical, you would look at the order book
and determine how much it will cost you to convert bitcoins to USD (or whatever currency you are considering).
Two things worth noting - this method could actually be higher than the geometric mean (if the current price is close to the high of the day). And the conversion rate really depends on how many Bitcoins you are converting. Just consider this: If it's 1 BTC, you are going to get a higher in USD than if you are trying to convert 10,000 BTC.
So look at the order book chart linked above, and look up the number of coins you are trying to convert on the side (Y) axis. Then slide your finger over to the orange line, and that will give you a minimum price. And where the orange ends (to the right a little bit), that will give you a maximum price. Look at the area that is contained in the graph of orange, between the minimum and maximum, and take the arithmetic mean of the minimum and the maximum. And that's your answer. (of course, you can figure this out mathematically, and not just by eye-balling it.)
That is the price that you KNOW you can offload the coins for, at that moment in time.