Todd Mitchell
@Christopher - first, thanks for the kind words on my mix!
First off, although there's good merit in knowing the guidelines for mastering levels, I'm not sure I'd be completely focused on picking a target LUFS. In fact, depending on the dynamics of the track, it's important to realize that different songs will be "scored" differently. I strongly encourage you to check out this website for evaluating your master:
https://www.loudnesspenalty.com/#
One really valuable lesson will be in seeing the spread between the services. Over time, I have discovered that both how I mix and the track itself will have a notable impact on how much a given service will turn it down.
In my opinion:
1) Optimally, no service will suggest the song should be turned up
2) Songs with a lot of dynamics and/or one disproportionately loud part will be more heavily penalized on some services. For my mix of this tune, the difference between how much Pandora will turn it down (0.9 dB) and Apple (-3 dB) are very close. This means that my mix probably has less dynamic range than it could, but also means that head-to-head with something pop/R&B/metal/hiphop, I should be right in there - no one should reach for a volume knob. If the spread between Pandora or YouTube and iTunes were big, I'd have to be sure I was happy about how dynamically different the loud and quiet parts of the song are. 