Suggest you look at what the baffle tube diameter is which is reported to be 2.1" Now treat that as a 2.1"field stop 14" or so inches in front of the eyepiece. You will find that you edge illumination is exactly 0 for the 3" 30.... Sorry to be the bearer of bad news. The visual back opening on large cats is irrelevant, its the baffle tube that counts.
I am happy to be the bearer of good news. That is not right.
If you have a 2.1" (53mm) baffle, obviously you do not get 0 illumination at 52mm. If fact, if placed at 14", I estimate 70% illumination.
Did you really mean a 53mm baffle blocks all the light of a 52mm circumference?
Also my info is that 2.1" (53mm) is the corrector CA, which is placed 7.4" (188mm) from focus (ideal 146mm back focus).
The baffle front ID is 2.25" (57mm), placed at 15" from focus (ideal 146mm back focus).
I estimate that to give about 75% illumination at 52mm circumference. Error margin 5%. So at least 70%.
Official numbers from Celeston (white paper) for C14HD (and C11HD) are 100% at 16mm and 83% at 42mm.
On a side note, 2007 Ken Hutchinson analysis of the standard C14 gives 73% at 52mm circumference.
Finally, the visual back opening is not irrelevant. Both the baffle and the opening must have the proper size. One without the other will not work. What you probbaly meant is that it is irrelevant if larger than a minimum size, but that is true for every baffle regarding vignetting.