Hi andqui,
the chance for an offset is given by
number of entries with the offset / choosefrom value
The value of choosefrom is 20 in your case, therefore each entry corresponds to 5%. If you had only the line 0_Dial0|RealVal=2.7,22.7 the torpedo would run at the correct depth with a chance of 95% and with 5% 2.7m below.
I think in SH3 (in contrast to SH4) the chance of prematures/contact duds is hard-coded
Cheers, LGN1