Id love to contribute my data, but I hardly believe it's accurate. I was having extremely good luck initially, but now, it's been taking me much longer to get a shiny via MM. What is the rate expected to be if you have the shiny charm along with MM? Here's my progress, but you can just disregard it for the data if it seems too general. I continued hatching the rest of my eggs after finding the shiny.
Sandile x1 - 180 eggs
Scyther x1 - 410 eggs
Murkrow x1- 10 eggs
Noibat x1- 390 eggs
Gligar x1- 30 eggs
Frillish x2 - 200 eggs
Porygon x1- 300 eggs
Cleffa x1- 450 eggs
Skarmory x1- 920 eggs
Vullaby - 800 eggs
Things started to get unlucky for me in the end. I'm still going for vullaby. Once again, this might not be accurate, since I got some eggs while switching stuff around, but my rate so far when I was seriously using MM is 1 shiny per 339 eggs. Which i still believe is pretty high. Why are vullaby only female? It sure makes it hard to pass egg moves.
It could be worse though, last gen I tried MM, and I got a single tynamo in over 2000 eggs, and it had a meh nature.