Shimano DENDOUMARU 4000HP Instruction Manual page 16

Zero-setting
To reach the rack exactly.
To improve your catch
Zero-setting
result, it is essential to
get to the rack exactly.
To do that, do zero-setting.
Zero-setting means setting of 0-Meter
16
(Always do it before you start fishing)
when the hook is on the surface of
water.
By doing zero-setting, the rack depth
can be reached exactly as the rack
position depth is displayed.
When the hook is on the surface of water, do the zero-setting