can any one explain calculation of sample content for water determination by using the USP method

FCV / KF

where

F is the water equivalency factor of the Reagent, in mg per mL;
C is the used volume, in percent, of the capacity of the buret;
V is the buret volume, in mL; and
KF is the limit or reasonable expected water content in the sample, in percent.

This is the formula for "how much weight(mg) of the sample to be taken ?

The ultimate reason for this formula is : Take more weight for consistent results. Thatâ€™s it. Nothing more than that.

Example:
you have an instrument with 10 ml burette capacity.

F=6.000 mg/ml(KF Factor )
C= 10% (1 ml of KF Reagent is exptected to cosume for the analysis)[
V=10(Size of the burette)
KF= Limit(specification)= 0.5%
Now,

FCV /KF
=6 x 10 x 10 / 0.5
= 1200 mg
=1.2 g

This is the way to fix the weight of the sample required for determination of water.