The shiny odds you're hunting with don't matter at all when finding the odds of finding a shiny of your target vs a phase. The only thing that we would need to calculate your expected number of phases is the odds of encountering your target (shiny or not) vs anything other than your target.
From your question, it sounds like that, out of an encounter pool of 2 Shelgon and an Amoonguss, you've gotten shiny Amoonguss 19 times in a row, rather than your intended target of Shelgon. Considering this is about a 1 in a billion chance, I'm assuming this probably isn't the actual case and I'm misinterpreting something, but I'm having a hard time figuring out what the actual situation is.
What other Pokemon are you encountering other than Shelgon and Amoonguss (your secondary target?), and what are the odds of encountering all of them? Can you give us the encounter table for the area you're hunting in? If not, there are a few other ways we can try to answer the question, but I'm going to need a bit more context of where you are, and in which game.