Uncertainties Calculation
If we have 2 variables 
with absolute uncertainties(+)
$A \pm \sigma_A$ and $B \pm \sigma_B$

(+)  expressed as standard deviations, but applicable to confidence intervals as well.

Then $C = A \pm B$ has uncertainty 

$$\sigma_C = \sqrt{{\sigma_A}^2 + {\sigma_B}^2}$$

For $C=A/B$ or $C=A\cdot B$,

$$\sigma_C = C \cdot \sqrt{\left(\frac {\sigma_A}{A}\right)^2+ \left(\frac {\sigma_B}{B}\right)^2 }$$

Let $A$ is content of cassein in 100 ml of milk. 
Let $B$ is the actual volume of used milk. 
Let $C=A\cdot B$ is the mass of  cassein in the volume of milk used in analysis. 

Then
$\sigma_C$
is the estimation of the error of the amount of cassein (known as the result error)
$\sigma_A$
is the estimation of the  error of the cassein mass in 100 ml of  milk (unknown)
$\sigma_B$
is the estimation of the  error of the milk volume  (1 ml )

The error introduced by scales can be neglected, or introduced as additive variable $D=0$ with absolute error estimation $\sigma_D=0.001~ \text{g}$.

Another, the biggest  factor is the used analytical method with its bias $E$ and error $\sigma_E$.

$$\begin{align}
C &amp;= D + E + A\cdot B \\
\sigma_{AB} &amp;= A\cdot B \cdot \sqrt{\left(\frac {\sigma_A}{A}\right)^2+ \left(\frac {\sigma_B}{B}\right)^2 }\\
\sigma_C &amp;= \sqrt{{\sigma_D}^2 + {\sigma_E}^2 + {\sigma_{AB}}^2}\\
\sigma_{AB} &amp;= 2.4 \cdot \sqrt{\left(\frac {\sigma_A}{2.4}\right)^2+ \left(\frac {1}{100}\right)^2 }\\
0.13 &amp;= \sqrt{{0.001}^2 +{\sigma_E}^2 + {\sigma_{AB}}^2}\\
\end{align}$$