There is a formula that I used (key word used) to know that would figure out what your true depth was, it will all depend on the swayback (line angle) of the downrigger wire. What I try to do when blackmouth fishing is to keep the downrigger ball within 10' to 15' of the bottom, whatever that may be. I will bounch it off of the bottom on the first set and continue to check it that way if the bottom conture is flat, or re-adjust if it changes. Hope this helps ya out.
Good luck and tight lines, Jeff
PS,
If anyone remembers the formula would you please post it.

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