In retrospect the photographer has three critical areas in this objective that need certainty because in 20x24 format mistakes are very costly. First he needs to verify the correct base exposure which seems anomalous to me as stated. Spot meter caucasian skin and place on zone 6. The second driver in this instance is he has a barrel lens and needs longer exposure times he can manage properly. I would shoot for about one second because that is the baseline for this application. Maybe someone has skill to pull off a half second with a barrel lens -then I tip my hat to them figuratively speaking. Lastly as I previously stated reading an easily seen square wafer at the plane of focus with a proper scale on the ground glass is uniquely simplistic and takes all of the "though" gymnastics out of the process. The scale immediately tells the photographer the bellows correction that is optically in play - any lens, any camera, and any magnification. Been using this for years on 8x10, 11x14 and 8x20 and it has never let me down. If there is one thing I have learned with ULF and that massive expanse of ground glass that needs considerable time to ensure it is all dialed in is "keep it simple" because while the images it produces are the reason we go down this road, the chances for little errors that can muck up an image increase proportionally with format size.

Hoping the photographer was able to get his shot.