Ackermann is basic stuff on cars.....

When you turn a car, the Ackermann setup makes the inside wheel "pull" the car into the corner, as the car body rolls the load is transferred to the outside wheel, it then takes over the turning of the car and "pushes' the front of the car around the turn.

To better simplify it.....
if you draw a circle and place the car's outside turning wheel on it, then the inside turning wheels path is on a much smaller radius'd circle.... Ackermann counters that by making the wheels turning rates differ to allow for the difference in the size of the turning circles of the 2 front wheels.

and yes there's a formula to work it out for the best angle desired in a car....... the formula works for all wheel based variations in the different sized cars we drive from a tiny "smart car" to a limo...

but for boats.......

As Scarecrow said we have other forces at work as we turn the boat, we also have the "steering" at the back of the boat where-as a cars is up front.... ever tried steering a car in reverse at speed ?????
The Ackermann principle for boats and cars is basically the same, the inside hull actually travels on a slightly smaller radius'd circle than the outside hull.


Depending on how sharp you "steer" the boat around the marks will have a big impact on the Ackermann amount you "need" to have......
If for example you slam the tiller across until it simply wont go any further and the boat speed isn't that great then you'd probably want a large amount of Ackermann
&
if for example you take big arc turns and rely on keeping the boat speed up as you turn then a small amount (if any) of Ackermann is desired


So if you slam the tiller across then you are doing small turning circles, and hence need the inner hull needs to turn tighter
&
big arc turns means you have a large turning circle so the need to turn the inner hull is less.



Hope you understand it all so far .....




So.......
lets put 100mm of Ackermann into the boat, it will near turn on a dime but will be as touchy as hell to keep going in a straight line, the slightest movement of the tiller WILL result in the boat "turning"
as you decrease the amount of Ackermann the touchy steering dissipates as does the sharpness of the turning....

Its a trade off as to which way you want it to work but most importantly, it has to suit YOUR style of tacking


& just for the record.... Twice Shy has 55mm of Ackermann over a distance of 380mm from the pintles
It turns nicely until you reef on it at speed, then its like slamming on the brakes and boat speed is lost instantly. The up-side is it can round a mark in a blink of an eye and tacking is a breeze.
With Jurassic Karp I intend to lessen the measurement to ~30mm as its a faster boat and will need a larger turning circle to keep the boat speed up.



The other side of it is...
if you've got dead straight rudder arms and the boat 'handles" the way you like it..... then why change it



cheers