How do I calulcate the amount of concentrated stock solution to add to get the correct dilution?
For any question regarding volume and concentration, you should consider the equation $C_1V_1=C_2V_2$. We can use it to derive a general equation for the addition of concentrated stock solutions. We want to find the volume of stock solution ($V_1$) to add so that its final concentration is 1X. Here's what we know:


The initial concentration of stock solution, $C_1=X$
The final concentration of the solution, $C_2=1$
The volume of solution we are diluting the stock into, $V_s$


So the equation becomes:

$$XV_1=V_2$$

We don't know $V_2$, but we do that $V_2=V_1+V_s$. Subbing that in:

$$XV_1=V_1+V_s$$

Solving for $V_1$:

$$XV_1-V_1=V_s$$
$$V_1(X-1)=V_s$$
$$V_1=\frac{V_s}{X-1}$$

Then you simply need to sub in your values:

$$V_1=\frac{25μL}{6-1}$$
$$V_1=5μL$$

So you should add 5 μL of your 6X loading buffer to the 25 μL solution to get a 1X final concentration.