I spent a lot of time on this issue last week. I have to say I have no idea how to master. ha! So I used an aid.
I sent a track to Landr for mastering then imported it into my master project to see if I could match the loudness.
I did many tests and the only one that got me to the level of the Landr mix was putting all my mastering plugs on the track and didn't use a buss. I set the meter to -3dbs, then exported an mp3 and wav file. I then imported them back into the master to match to the Landr... Bingo!
Well it's what I did... :) lol