FOCUSER BAFFLE VIGNETTING
The focuser baffle can cause more vignetting, see the figure below;
The red lines show the lightcone for the fully illuminated area. The baffle has an hole that lets only this lightcone through.
The blue lines show the lightcone for a pinpoint object outside of the fully illuminated area. The A and D line show the full lightcone as reflected by the primary (assuming there is no vignetting for this object by the tube). Due to the size of the secondary, the part of the lightcone between the lines C-C2 and D is cut off. This was part of the tradeoff when the size of the secondary was chosen.
However, the part of the lightcone between the lines B and C-C1 is now cut off by the focuser baffle. (Note: that the line B, after reflection, is horizontal is pure coincidence)
It is for the above reason that you may not want to use the diameter of the fully illuminated area when determing the size of the baffle hole. To be save, you could use the size of the field stop of the eyepiece.