According to This tweet, "So 65521/65536 wild encounter shinies will be squares, and 15/65536 will be stars."
That is about 99% of random encounter wild shinies will be the square shinies, and the remaining about 1% would be star shinies.
According to Here, marks can range form a 0.1% chance to a 4% chance without the Mark Charm.
So, the odds of a shiny are 1/4096, which is about a 0.02% chance.
This leaves a low and high of about 0.002% chance and a 0.08% chance, I believe
Congrats on your marked shiny, but unfortunately you got the higher odds of the two shiny types. Also note, they are reversed for breeding, and you cannot get marks by breeding.
If anyone wants to fact check my math, feel free, I am by no means the best, but here's all the numbers needed, and all around a nice average.
Check my sources for more detail :3