Closer your engine, for sure i would be placing right next to the vertical tape.

 There is a rib running inline with the vertical tape like in the 2nd pic it stops about 1.5m from the transom and the one in the pic stops about .5m from the transom 

 I be would be going inbetween those strakes but on the starboard side if standard rotation prop. You want to be on the props downward swinging side 

 I have duo props so wont matter on what side as far as I'm aware and it would prob get damaged on the starboard side getting in out of the boat on the ladder.

What advantage would there be in going between the strakes to where I have it in the pic just trying to understand  is it better lower in the water? 

Where your holding it will be turbulent with the flat spot in the hull. Go in between those ribs and drop it low, usually there will be rooster tail from the larger trannies see how your reading goes and if requiredyou can make or buy a spray deflector to suit

 and I'll add the further out to the edge you go your sounder gets affected by when your drifting and the boat rocks you'll  get bubbles under it as it may come out off the water and when you turn in your case to starboard the tranny comes maybe not out off the water totally but closer too the edge and it will again affect the reading. Go in between those ribs. 

 Great thanks for the feedback

When you go out next sit on the plaining speed and see where the clear water is out the back of boat. Should be able to see where it is clear and where the white wash is. I suspect will be closer to the leg will be the cleanest water. Next is angle, need to get angle placement right, Airmar have a good installation guide, follow it and you shouldnt have any issue.