...just remember, the true application of a sonotube is to "form" the portion of the pour that has no form. If given an option to push it all the way to the bottom of the hole, or only half way, the better choice is half way and allow the the concrete bearing mass to form and bind with the subsoil below. Use a 1 x 2 firring strip, running it up and down methodically after the pour right into the concrete, to minimize air pockets and to assure a homogeneous pour; about 20 strokes up and down in varying points across the entire concrete area.
Are you going to use post anchors?