Welcome to MB?
We tried the iPhone app last September but it was very spotty as to accuracy. I’m guessing apps like that work better in dense crowds (where there is a lot of iPhone input).
Never been there in early Dec., but based on listening and reading I imagine lines aren’t historically that bad. Getting the Unofficial Guide is great advice. Their online link above gives you heavily researched historical crowd levels. I would say levels at 6 or below mean good times. Also, know like the rest of us here, the morning EMH park will often be the most crowded park of all for most of its day.
However, even UG says their crowd level chart has less bearing on wait times than picking a good touring plan from the book or site and following it.
We’ve followed the touring plans in the middle of summer many times and rarely waited more than 15 minutes.
Hope this helps and have a great trip!