What should the toe in setting be for a drop tube axle I have on my A low boy with cross steering.

The reason I ask is I originally set it up by eye and tape measure and the car has driven fine but of course the tyres are scrubbing so I had it checked and it was 2.5 degrees toe out.
I have set it at zero but the steering really shimmies when the offside hits a bump ( we have a lot of them in somerset) at about 40mph it does not clear I have had to slow right down until it stops.

I have checked that I have not lost a balance weight and there is no apparent component failure giving extra play.

There is a panhard rod fitted but no steering damper.

Any suggestions as the car has become un drivable. The easy answer would be to put it back how it was but then I have not learned anything.

Thanks

Rich