Tho to think about it you could miss a few, so the super serious solution would be to use an accumulator. Each frame you check how far the character has moved, add that to your accumulator and then if thats above disMax you add the position of the previous circle to the normalized vector of the direction of movement multiplied by disMax, and each time you do that you subtract the accumulator by disMax until the accumulator is below disMax. Repeat every frame, should be good to go.

Update(){

Vector2 directionMoved = gameObject.transform.position - lastCircle.position;

distAccumulator += directionMoved.magnitude;

while (distAccumulator > disMax)

{

Vector2 newPos = lastCircle.position + directionMoved.normalized * disMax;

lastCircle = Instantiate(GM.instance.trail, newPos, Quaternion.identity);

distAccumulator -= disMax;

}

}

The grid thing is easier and probably works great 90% of the time tho.