Do you have the shiny charm? If not try to work on that. Before I had mine I only got shinies through chain fishing and wonder trade. But since I got it my shiny luck has improved significantly. My first attempt at Masuda Method was with a Phione and it took me about 450-500 eggs to finally hatch one with the shiny charm. Not too long after though, I was IV breeding HA Totodiles and unexpectedly hatched a shiny in my first 10 eggs. It was a pleasant surprise. Since then I've just had random horde encounters when EV training, which never happened when doing so without the shiny charm. As far as using the DexNav goes, I believe the odds are about the same as with the MM. Oh and FYI, breaking a DexNav chain won't reduce your chance of encountering a shiny. Just remember to have your synchronizer with proffered nature. As far as which is better between DexNav and MM, it's basically up to you. MM is more passive but allows you to have better control over IVs and nature. Save up eggs, hatch, rinse repeat. Plus you'll have a bunch of breedjects to wonder trade to collect pokemiles. On the other hand, DexNaving I feel is more proactive in the sense that you feel like you're trying to actually hunt down the elusive rare shiny. You have less control over IVs, but the synchronizer should help with nature. You also have the chance to get one with some fun egg moves. I think it's more fun that MM. So with that being said, it all comes down to if you want to have a more reliable way to have a competitive Eevee, or just want to have one just to have it and show off the pretty shiny.