Get the eyepiece's field diameter from the manufacturer's specification. Barring the spec, measure the field lens diameter at the bottom of the eyepiece barrel.
For a complete picture, the chart includes light lost from the diagonal obstruction. Larger diagonals improve off-axis illumination but cost light across the entire field.
Experienced observers agree that a star's light loss of less than 0.3 magnitude is not noticeable. For extended objects where both the background and object drop equally in magnitude, the contrast or ratio is unchanged, meaning that the eye's ability to detect the object remains largely unchanged. For instance, compare a 10 inch to a 12 inch telescope. Experienced observers can detect a small difference at the eyepiece. The magnitude drop is 0.4. The difference between a 11 inch and a 12 inch telescope is not readily apparent. The magnitude drop is a smaller 0.2 magnitude.
Diagonals act to degrade optical performance, the larger the diagonal, the worse the degradation. A one-third obstruction, a much larger ratio than any visual Newtonian uses, degrades the optical quality by one-sixth wave (less than what is commonly quoted - see http://www.telescope-optics.net/obstruction.htm for an explanation). Changes less than one-eight wave are very difficult to see.
Don't forget to baffle the diagonal - see baffle.htm.