I don't know about the Michigan Prediction program being the "favored" site of U.S. users. This is the first time I've even heard of it. I have always used the HAB-HUB Flight Prediction site and been very happy with its layout and predictions. In fact, I'm usually within 3-5 miles of the predicted landing site with every flight.
A couple things you might consider to make it a bit more user friendly, based upon my initial introduction to it:
1) What size tank of helium are you assuming people are using? I use "T" cylinder/300 cubic foot tanks when I fill. Is your model based on a "K" cylinder /200 cubic foot tank? It would be nice to let the user know.
2) Payload weight — the Cambridge program asks for weight in grams and I've adapted to that pretty easily. I find that it is a bit more precise than using ounces and pounds.
3) Your program allows predictions only 3 days in advance compared to the Cambridge program that goes out a full 7 days in advance.
4) Your program uses top-of-the-hour hourly launch times where the Cambridge program allows you to enter virtually any time during the day.
I'll give your program a try on my next flight coming up soon. It's always nice to have more than one resource to reference. I give you guys a lot of credit for tackling something like this. I have no idea how you do it, but in my humble opinion, these flight prediction programs are "magic"!