Theoretically it should be:
STF = FL * FR * (MTS - SD) / (FL - FR * SD)
(I cheated and used an online tool to help since I am lazy)
MTS = 48 mm (this could be the thread size of the filter size, whatever is the smallest)
And using the example values above of
SD = 43.3
FL = 1000
FR = 4
Would give a STF of:
1000 * 4 * (48 - 43.3) / (1000 - 4 * 43.3)
= 18800 / (1000 - 173.2)
= 18800 / 826.8
= 22.74
If you've got your filter more than 22.74 mm away from the sensor when using 48 mm threads, you will get vignetting. A quick reality check shows that the result would be negative for filters of less than 43.3 mm in size, which is what you would expect (and therefore would automatically be vignetting no matter the scope).
Please check the math and numbers.
