I teach drawing and my students are required to measure alignment of objects using their pencil as a straight edge whilst looking through one eye. Some students

I teach drawing and my students are required to measure alignment of objects using their pencil as a straight edge whilst looking through one eye. Some students have trouble with this and have to place one hand over there non dominant eye. Since I lost my left eye I have no trouble looking through only one eye. However, I have acquired plenty of troubles with balance.