Tears (not mine)

not sure, could use more information, but tearing most happens when the outer skin of the loaf is uneven in moisture content.  A weak spot caused by lack or too much moisture in that location.  It looks like the tearing is happening where the banneton meats the dough, and a gap opens.  Try dusting the bottom exposed dough as much as the banneton before it rises.  Try using a cloth instead of plastic to cover it.  Relative humidity of the room would affect how the skin is forming on the dough as it sits and rises.  Too dry makes a tough crust, too wet, steam will expand the loaf where it can stretch most easily.  The places of least resistance.  Scoring deeper may focus expansion to the cuts.  It is really not such a big deal, the loaves still have nice bottoms.  You could also try parchment or more flour under the loaf so it doesn't stick to the stone at all.  It can be this clears up with more stone use.

When the loaves are ready for the oven is a tricky matter and I'm faced with that decision every time I bake a loaf.  I tend to rest my whole hand on the loaf, pressing firmly but gently into the dough with my finger tips.  I oil, flour or wet my hand so it doesn't stick and check on the loaf often.  Do it when you first shape it so you have a basis for how the loaf changes.  Get a good look at it and take a digital picture so you can compare later if needed.  Feel for the sponginess and firmness of the loaf.  Feel it expand under the surface, the gas filling the loaf.

As it rises and expands, check it again, often, and feel the loaf without deflating it, press your whole hand firmly onto the loaf and use your other hand if needed. 

A gauge can be simply made with a little ball of dough.  Find a small narrow glass or something like a film container and press a little ball (of pinched off dough when you're shaping the loaf) into the bottom and make it as level as you can.  Mark the level with a rubber band or tape and mark the dough when it would be "double."  Depending on the recipe, you will want to wait until it doubles to bake or almost double or whatever.  This gives you something concrete  to follow, whereas a banneton (bowl shape) is hard to read when it is almost double or doubled.  Often times the dough rises more than one thinks!      

If you combine both these methods of determining rise, you are teaching your hands and brain when the dough is ready to bake.    :)