Q:

What's the CR of this group of poachers?

The PCs are a 5th-level party that includes a two-weapon-fighting ranger with the feat Cleave and a druid that usually prepares the spells entangle and charm animals.
The foes are a family of poachers. It should be a sort of boss fight to help the PCs look cool.
But I'm having trouble analysing the encounter's Challenge Rating. The rules are unclear what happens when mixing more than 2 types of creatures. There's a column for mixed pairs, but no column for a mixed group of four.
So far, the group of enemies includes the follow:

1 human fighter 2 (CR 2)
1 human ranger 2 (CR 2)
4 human warrior 1 (CR 1/2 each)
3 dogs (CR 1/3 each)
1 riding dog (CR 1)

My math says that this is a CR 6 encounter, but that seems high for what looks like a rather weak bunch. Am I even relatively close? What is this encounter's actual CR?

A:

Your group is EL6 which is generally considered above average for a party of four characters of level 5.
To calculate group EL you can use one of two methods.

Use encounter calculator. There are plenty of them on the net. This one, for example.
If you don't want or cann't use one of them for some reason, you can calculate it manualy. The DMG has not only table 3-1 (DMG, p.49) to help you but also a very usefull phrase above on the same page.

In general, you can treat a group of creatures as a single creature whose CR equals the group's EL.

Thus you have 
\begin{array}{lll}
\text{Second level human fighter (CR2)} &&\text{CR2}\\
\text{Second level human ranger (CR 2)} &&\text{CR2}\\
4 \times \text{First level human warrior (CR 1/2 each)} & = & \text{CR2}\\
3 \times \text{dog (CR 1/3 each)} & = & \text{CR1}\\
1 \times \text{riding dog (CR1)} & & \text{CR1}
\end{array}
Two CR1 monsters in turn make up CR2 together.
\begin{array}{lll}
\text{Second level human fighter (CR2)} &&\text{CR2}\\
\text{Second level human ranger (CR 2)} &&\text{CR2}\\
4 \times \text{First level human warrior (CR 1/2 each)} & = & \text{CR2}\\
3 \times \text{dog (CR 1/3 each)} + 1\times \text{riding dog (CR 1)}& = & \text{CR2}\\
\end{array}
And \$4 \times \text{CR2} = \textbf{EL6}\$
Do have in mind that the real difficulty of any encounter depends highly on many factors like the battlefield, homerules, player's experience and so on. So be ready to modify the final EL according to the circumstances. 

