Brett, the whole point behind Ackermann steering is the rudders don't stay aligned when they're turned. The outer rudder actually turns less. On a car this reduces tire scrub, on your boat it means you generate similar turning forces on each rudder despite the outer rudder is travelling faster through the water as the boat turns.

So to check your alignment you need to have the rudders aligned fore and aft.

On a car there is a particular easily defined Ackermann angle which keeps all four tires tangent to curves centered on the same point when turning.

But your boat doesn't turn around a single point, it slips sideways and has a different centre point for any speed and rudder angle so there is no "perfect angle". Which is why people aren't rushing to answer your question. On a boat like a mossie, people will normally put about 75-100mm of inset on the tillers. Less will have less resistance if making small corrections at speed more will have less drag when turning.