I think this is an indication that your 6th grader with dysgraphia also has problems with visual processing. Finding out exactly what is going on with the visual system might be more of a help, so that you could really design target interventions, or at least get accomodation so that estimating visual angles isn't required. This is purely a visual skill. That said, getting one of
these and spending some time playing with it might help with the "estimating angles" problems. We found a set of
these shape blocks pretty useful for developing a feel for different common angles and also for playing with the idea of symmetry. Getting a plain piece of mirror tile so the edge of the mirror can sit flush against the page and playing with that to see how reflection through a line works in practice might be helpful, too.
