There are two variants - one that allows for your fire warriors to advance while remaining well protected, and one that keeps them static.
As soon as I have access to Vassal 40k I'll try and do more detailed work on this.
The Basics
The main thing about the Sea Turtle tactic stems from the simple fact that, as a transport, there's little use for it once its deployed your fire warriors. On top of that, if you're running a Pathfinder team then you have this giant hunk of plastic that you really don't need except to provide mobile cover.
The mobile cover tactic is where this comes in.
Effectively you can have a unit - most notably our squishy fire warriors - that starts deployed just behind the devilfish. The fish moves at an angle to allow the fire warriors to advance forward and then fire, then move back into a position to provide them cover. Alternatively you can simply move the devilfish behind the fire warriors then use your flat-out move to return it to its original position.
The Math
It's actually really simple when you break out your high-school geometry book (or google).
We want to open up 6" of space for our fire warriors to advance. That means our move needs to be broken up across two moves that each open up 3". In addition we want to open up as much space as we can for them, meaning that we move our devil fish as far as we can.
Finding the Right Triangle
The key here to figuring out your movement is thinking of it as along a right triangle in each phase. You have one side that is 3", and the hypotenuse (long side) is the movement your vehicle makes. That means you can also calculate how much space you're opening up.
It's as Easy as 3-4-5
If you really remember your Pythagorean Theorem (a2 + b2 = c2) and game with a calculator you can quickly figure out your movements. However there is a quicker method.
Remember - we're wanting one of the two sides of the triangle to be 3". If we do our movement at 5" that will put the center point of the devilfish 4" to one side of its starting position, giving the fire warriors a fair amount of space to move.
Repeat the same process and it returns to the same point relative to the fire warriors it begn its movement with.
Behold the handy-dandy diagram.
Now - of course there may be terrain to consider. You can also actually do your movements in different angles to allow you to adjust course rather than advance in a straight line. This is just a basic overview of how the system can work to allow you to provide basic cover to your fire warriors while they rapid fire their way up field.
Remember - we're wanting one of the two sides of the triangle to be 3". If we do our movement at 5" that will put the center point of the devilfish 4" to one side of its starting position, giving the fire warriors a fair amount of space to move.
Repeat the same process and it returns to the same point relative to the fire warriors it begn its movement with.
Behold the handy-dandy diagram.
Now - of course there may be terrain to consider. You can also actually do your movements in different angles to allow you to adjust course rather than advance in a straight line. This is just a basic overview of how the system can work to allow you to provide basic cover to your fire warriors while they rapid fire their way up field.