I built my own, too, along with a pre-chiller, and I won’t cool a wort down without the pre-chiller, especially in the summer or a lager. I used a corny keg to wrap my pre-chiller (50’, 3/8"), and it fits perfectly inside a five-gallon, round Rubbermaid water cooler. I used a large, empty nitrogen tank for the wort chiller (25’, 3/8"). I disagree with Nighthawk, as I timed several batches, and using the pre-chiller cooled my wort down around 30% faster.
Of course a pre-chiller is going to work faster than no pre-chiller, I don’t think that’s what he meant. You can make much better use of the ice by pumping ice water directly through the coils than cooling off your ground water, which is a continuous additional heat source.
If you don’t have a corny keg (like we didn’t) we actually used a paint can, which was the perfect size…
When I first started brewing I had an immersion chiller. At some point I received a counter-flow chiller from another club member at a really good price but I kept my old immersion chiller as a pre-chiller that I would put into a bucket full of ice. I would run the hose water thru the pre-chiller ice bath and then thru the counter-flow chiller. In the summer time I could get wort into the fermenter at 60 to 65 degrees in 15 minutes or so. In the winter when the ground water temps were lower I could get the wort down to 50 degrees if I ran it too slow. Of course in Houston in the summer time you have to use the ice bath as the ground water temps are in the 80s. 
Somehow when we moved into our new house my immersion chiller went missing and I haven’t had a chance to replace it. What I have been doing lately is packing my fermenter into a square ice chest packed in ice and running thru the counter-flow chiller into the ice packed fermenter. Works pretty well but I really need to replace my pre-chiller this summer if I plan to do any brewing.
Or you could spend that money on a cheap submersible pump instead and have a much better ‘prechiller’ solution, and the guts for an automated keg/carboy washer to boot.
I have found a post chiller works best with a CFC. I put a SS coil in an ice chest and runt he wort from the chiller through the coil and into the fermenter, it goes in at about 52 degrees.
What you are doing works well also because you can fit your fermenter in ice, mine won’t so a post chiller is my best option.
