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Middle of the tarp. Heres the pole mod

As Brian said, you can only do so much. Try either connecting the ends to the trees without a continuous ridgeline or try an over the top ridgeline. This may help pull the material up a little more...

Hard to tell if you did this but for the corner tie outs you can also make sure it is staked out so that if you were to draw an imaginary line the cord would be centre of the triangle of the corners...