Should I have given it a greater axial angle than a mere seven and a half degrees?

The two rows of three cylinders are arranged at a 15 axial angle from the crankshaft.

If it is measured by means of eye-piece micrometer, the optic axial angle of the mineral can be found by a simple calculation.

The distance between the axes as seen in the field of the microscope depends partly on the axial angle of the crystal and partly on the numerical aperture of the objective.