Even with the Mirror Lockup ticked it still deducted 4 sec from the exposure.
I also trialled it with the ICNR. I took 2/35sec shots, each missed 4 sec on the light so were only 31" long, and then the ICNR went for 44".
I guess though there is the time required to write the light frame to the card, the time to take the ICNR and then the time to apply it to the light. But if the ICNR frame was the full 35" then it would have been an incorrect calibration.