Sorry, yours question and basic calculations aren’t very clear.
Please read also the basic information on Plated available here:
http://www.925-1000.com/a_platenumbers.html" onclick="window.open(this.href);return false;
There are the basics sufficiently explained.
Plated cutlery include ‘salt spoons’ of about 6 cm length to ‘soup ladle’ or ‘punch ladle’ of about 25 to 30 cm length.
‘DWTS’ is a sign of amount of Silver (.999 fineness) used for both sided galvanic processing of 12 items of cutlery.
‘Warranted 12 DWTS’ is 12 x 1.55517384 grams Silver of .999 fineness.
Around 18.66 gram is normally used for both sides of 12 table forks
or table 12 spoons (soup spoons).
Knife hafts haven’t ever the same amount of surface as the related forks or spoons.
So on hafts surface is normally much less silver deposited.
Some body has long time ago already explained that too:
That there are also at least 5 qualities usual; ‘A1’ is the best or ‘Prime’ quality, ‘D’ the lowest.
12 large table spoons, quality ‘A1’ has 32 dwt., ‘A’ has 24 dwt., ‘B’ has 16 dwt., ‘C’ has 12 dwt., and ‘D’ has 6 dwt. Silver.
‘BBB’ Barker Bros., Birmingham:
12 Table spoons, large, quality ‘C’, 12 dwt. Silver
12 Table spoons, small, quality ‘B’, 12 dwt. Silver
12 Dessert spoons, medium, quality ‘A’, 12 dwt. Silver
12 Tea spoons, large, quality ‘A1’, 12 dwt. Silver
12 Sugar spoons, quality ‘A1’, 12 dwt. Silver
12 Butter knifes, massive, quality ‘A1’, 12 dwt. Silver