From those in the know (John F). Put I down the back as far as possible, and in clear water. By that I mean, nothing in front of it as far as hull rib's, water inlets, etc etc. make sure also there are no other transducers running the some of even similar frequency. Adjust your gain until you just cull the graining one the screen mid and top water. You want the tranny as flush as you can. Even the slightest air bubbles will give interference, so ideally, have it in nothing but solid water and you will get the best reading possible. 

I hope that helps. 

Why pop a hole in your hull when you can go a wet box?. I just put a 1kw into one of my cats hulls as there's not really a good amount of real estate on a cats bum for a freeking big transducer. Never have to worry about losing the bottom at speed too.  

https://support.garmin.com/en-US/?faq=w3OwmYsyl68Q1MvXnsXDv6

 Some hulls simply don't have the room. Not only sub 6m boats, either.  Absolutely impossible with my Reefrunner--fuel tank takes up all useable space for the aft 1/3 of the hull, apart from a tiny gap between it and the transom.  It was just big enough to get a thru hull B175m in there and secure it with the locknut, without touching the tank. Setting them up with regards to strakes and obstructions to water flow is not as clear-cut as it may seem either. The old "don't put it behind a strake" becomes meaningless if you have short strakes.  If you have ever seen an underwater picture of a hull at speed, short strakes have their bubble stream fanning out as they leave the trailing end--sit it right between two of those further back and you end up in the convergence of those streams, and your picture at speed will be sh*t, like mine is. And a wetbox wouldn't be any different in those circumstances.

I recently helped a mate install a TM265 (LH?) on his old 6m Westerberg, feeding an Axiom Pro 9 same as mine. I told him where to fit it, and the bloody picture at speed is brilliant.  

After 20 years of trying transom mounts, in-hull, and through hull transducers

I finally got a good picture up to WOT and rougher water :

got a through hull with the high performance fairing (and all the old holes fixed)

by a professional installer (Fibrelite boats in O'Connor)

No problem with the trailer or running up on to the beach.

My boat is different (UB 6.7) but also difficult for transducers so 

consider the through hull with fairing. There is probably a reason that most of

the professional boats have them.

 They get the transducer face out of the disturbed water running along the hull surface. They will cost you speed, for sure, and not all trailers will suit them, may need modifying, at least on the transducer side. The Westerberg I mentioned earlier had a faired transducer, old style, 50/200.  Had to build the trailer to suit. He's actually getting a much better picture at least on that hull, with the TM, go figure. And he's picked up a good knot. Some large faired transducers can cost you 4 knots on a fast boat, according to those who fit them. But are happy with the picture they get.