Neat idea, but I found easy mode trivial and medium mode completely impossible (had never heard of most). I think it needs better hints, or warmer-colder info or something. I think it would be more fun if it was a way to learn as much as a test of what you already know.
You should get a toast notification in the bottom right saying how many miles away your guess is from the correct location. I know there's no instructions, I'll add those soon, but also let me know if the guesses weren't telling you the distance correctly.
Oh, I never saw the toast as you have to knowingly submit a totally wrong guess to get it. Most people won't know how far distances in miles are (vs. km), but I guess it's just a number that goes up or down.