Just had a look at my 8 pole 400Hz filter with it set to kick in at DSP BW 900Hz to keep any DSP effects down and the AGC set to off. Did the measurement (using my FT1000MP MkV providing the sig into dummy load with wire to K3 antenna port wrapped round it giving S9 on the K3 meter. The MP has the high freq stab option fitted).
Using the dBV meter on the Elecraft display and resetting it to 0dBV on each measurement by reselecting it on the menu.
Initial results for -6dB points: -265Hz / +198Hz (meaning offset too high by approx 30Hz within 463Hz)
Adjusted 400Hz filter offset value to -0.03
Final results for -6dB points: -235Hz / +228Hz (meaning offset too high by approx 3Hz within 463Hz)
Note that the filter bandwidth was measured dead on 463Hz on both measurements giving me some indication of measurement accuracy.