All of you are right on all comments...
This is just a very quick, basic, NON-SCIENTIFIC approach that I use for an
approximation (guesstimation) of lure depth. There are a few charts that have been developed based on more scientific approaches...There are many things that effect the lures position in the water column.
DISCLAIMER:
(a little humor here for those that tend to miss these types of things) ... This method
does not factor in variances for changes in speed, direction of travel, wind factor, current speed, going with or against current, water density, barometric pressure, lure manufacturer, rod and reel manufacturer, time of day, sharp objects can cut you, hot items can burn you, if ya hat's turned sideways, have gold in ya mouth, and ya pants on da ground!