Yooper, your math is good (3@212 + 2@32 = 5@140). However, you are forgetting two points. One, ice from the freezer is more like 0-5f I think. Two, a lot of energy is needed to melt the ice (energy in the form of heat from your wort, aka "latent heat of melting"). Doesn't matter because you are still correct that it won't get you where you want to be.
Also, I have built an emersion chiller very similar to what is being described. All parts from Lowe's, total cost around 24 compared to LHBS at 70. Anybody else having problems with the copper to vinyl hose clamp connection? I'm afraid to tighten more for fear of crushing the copper, but I have noticed a very slow leak. Planning on making a solder connection.
One more note that I thought was cool and I haven't seen in the past - I wanted the cool to have a low profile so I could use it on very small brews (think yeast starter). I bent my 20ft into a double helix so it is about half as tall as other models I have seen. Probably took me 5 min of though plus 10min of bending instead of just 5 min of bending, but worth it IMHO.