Dive curve

I have a question on the dive curves. A jet 30 shows on the card 125feet out for 20ft depth. The bible says 90 ft out to reach 20ft. The discrepancy is about the same for a jet 40. I realize i can check this just by gettin into 20fow and seein where i start hittin bottom. I also realize it depends on the line, im using 20lbtest. Im using the big water edition for precision troll as the bible edition which pretty much is just for jets, dipsys, snap weights etc. Any advice from you pros? Thanks