Hi,
With the boom set so thats its flush with the front boom clamp i set mine so that the space between the rearmost clamp that secures the vertical fin and the end of the boom is approx 3mm. I generally set it so that its tight and there is some resistance in the drive system and then i will slightly loosen until the system feels smooth and then tighten up the bolts to secure un place.
In the picture you can see the gap between the rearmost clamp and the end of the boom:
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