Jorge, I'm with Caper go all the way out to the edge. The further out the less movement the tabs have to make to change your boat's leveling attitude. Think about it, you want the tabs to see full, level water flow/contact across the tab's surface. If you do it like in your picture the inside 3" will have an air pocket before the water hits the tab. Plus mounting to the outside you have the empty middle step to mount transducers. Mine are out there on the far outside. Good luck!!
Oh yeah measure many times to put your mind at ease and then drill like crazy!!
