Flat line vs inline boards

Hello, I would like to ask a newbe question. I have line counting reels and am learning the ropes slowly. I have learned alot from you veterans here on this forum. Thank you so much for taking your time to help.
I read where people say they are 40 back. My ? is, 40 back from an inline board would be at a different depth than 40 back from a rod tip 7 ft in the air flatlining of the back of the boat correct? How do you judge the difference if you would like to fish the same depth? I am using two offshore boards and two rods straight back. Any suggestions or help would be much appreciated.