Incredible. I made the attempt I've told about, screwing in the front nut of the fin head an headless screw, waiting to see it appear far from the hole of the board.
Surprise: the.. "headeless neck" of the screw was almost right under the hole!
After scratching my head for about five minutes, I then realized this.
The lower part of tuttle fin head is flat like the board hull and obviously at 90' from fin body.
BUT the higher part of a tuttle and deep tuttle fin is at an angle of what, about 15 degrees?
Then the two bolts-screw starts from their nuts and goes out over the top of fin head at 90 degrees BUT FROM THIS UPPER INCLINED SIDE! So they are naturally bent backward at the same angle!
So the screw heads over a flat deck like the one of the F158 willl ALWAYS be not flat and inclined backwards at about 15 degrees!!!!!!!
To avoid this and go at 90 degrees the holes over the tt and dt fin head should be INCLINED FORWARD at the same 15 degrees, but in this way the front screw would come out almost in the corner of fin head, not so good and strong.
And this with both the stock DT Drake Race 70 cm and the TT T-Zone 40/56 seaweed fin
Not having any problem with the Serenity I was wrongly thinking that the problem was in the board, but it's not, I have to say that pheraphs is a problem of the tuttle system. I'll check the serenity but I suppose that its hull is not flat, neither it's deck and so the bottom of its screw fin box (the so called drinks storage box).
I suppose that the TT system was invented in the years that almost every board has a stern width decreasing going back, and in this case it can be ok, but with a board like F158 that has parallel deck and hull.
Not to say that a one screw system like powerbox is not affected by these kind of problem, and this could be the reason that it has a wide concave in the middle.
Anyway with a somewhat flexible steel washer and a rubber spacer under it I've solved the problem, I'll use the T-Zone 40/56 that I've already bought for the Serenity but almost never used as I prefer for it the more extreme Select Delta Antiweed XXL that has an incredible 27,5 cm depth an allows me to ride on the shallowest waters (and wildest zones of my lagoon.