First off Barry Schiff is the man. I have played with this impossible turn a few times. My personal rule is: If climbing out at VY or slower (C172) and the wind speed is 10 knots or less, I can make it back to the airport if I react within a 5 second window after engine failure at 750 AGL or above. I always am on the lookout for fields to land on when I climb out from any airport and wouldn't attempt the turn at all if I had an atleast semi-suitable field within glide range. I saw in someones post earlier that a C172's glide ratio was 12:1. I think thats being a little optimistic (for the birds I fly anyway). In my opinion, for the math to turn out close to actual performance, a glide ratio of 8.5:1 or 9:1 would be more accurate. I would recommend just trying one every now and then in different conditions and see what you get. Only you can make the decision to turn back in the event of an engine failure, and while i teach primary students to never turn back on climb out, I believe its priceless to have the experience of experimentation with the maneuver so you will have an idea of what has to happen for YOU to make it back to the airport in a real situation.