Well done Harry Gibbon, who directed me to the Merchant Shipping Act of 1894. I have extracted the relevant bits from what is a huge tome....
Deductions for ascertaining tonnage.
In measuring or re-measuring a ship for the purpose of ascertaining her
register tonnage, the following deductions shall be made from the space
included in the measurement of the tonnage, namely:—
(i) any space used exclusively for the accommodation of the master; and
any space occupied by seamen or apprentices and appropriated to their
use, which is certified under the regulations scheduled to this Act with
(ii) any space used exclusively for the working of the helm, the
capstan, and the anchor gear, or for keeping the chart, signals, and
other instruments of navigation, and boatswains stores;
There must be permanently marked in or over every such space a notice
stating the purpose to which it is to be applied, and that whilst so
applied it is to be deducted from the tonnage of the ship:
The 66/60 therefore refers to the allowed deduction, not the actual.