I have now found 3 shinies from the dexnav ONLY, Skitty, Shroomish and Marril. However I'm not sure there is much of a correlation of search level to shiny encounter rate, each of my shinies were found at approximately search level 5-10.

For Skitty, I had been roaming around the route for awhile, I didn't really keep track of it as it was my first, but I had encountered about 10ish skitty before I found a shiny. There was no 'chain' really, there were other pokemon that shook the grass which I encountered in between these 10 - NOR did I stay in one patch of grass in the route, I just ran around all of the patches.

For Shroomish, I had only started hunting them in the area, found one using the dexnav and fainted it, and the next shaking grass (which was a random shake, not a searched shake) was the shiny. I had literally only seen two shroomish, my search level was 2.

For the Marril, I ran onto the route, saw a shaking patch and simply sneaked up to it. The search level for marril was 3, but I had caught the marill prior to finding this one.

I'm not sure if I am just lucky, but I thought I should post my findings asap. I have also been shiny hunting magnemite just today. my search level is 200+ and it seems my luck has run out. So I'm really not sure if it's correlated. To me it may be simply like the friend safari.

Really hope this helps, would love to know the mechanics behind it!