Hello,
I'd like to preface this with the fact that I'm incredibly new to Salesforce Development and most coding in general, actually. So please bear with me.
Ok, 
I'm trying to create a handy little time-saving flow-powered button to help people create a New Sales Order as quickly as possible.
So the button is located on the Account page, so this way, it already has all of the accounts information to enter into the Sales Order. So basically, the flow looks up the account, Creates a Sales Order, and then is finished. 
Initially I wanted the user to be redirected to the Sales Order they just created.

Initially, I had a functioning controller that looked like this:

<apex:page standardController="Account"> 

    <flow:interview name="New_Sales_Order_From_Account_Page" 

    <apex:param name="vExistingAccount" value="{!Account.Id}"></apex:param>

    </flow:interview>

​</apex:page>

Then, before seeing your post, I tried to do something like this:

<apex:page standardController="Account"> 

    <flow:interview name="New_Sales_Order_From_Account_Page" 
        finishLocation="{!URLFOR('/' + Account.ID)}">
    <apex:param name="vExistingAccount" value="{!Account.Id}"></apex:param>
        <apex:param name="New_SalesOrder_ID" 
            value="{!$CurrentPage.parameters.New_SalesOrder_ID}" />
    </flow:interview>

​</apex:page>

Which I believe to have turned out to be a disaster.
Then, after seeing your post, I tried this:

<apex:page controller="flowController">
<flow:interview name="New_Sales_Order_From_Account_Page" interview="{!flDemo}" finishLocation="{!prFinishLocation}" />

public class flowController{
public Flow.Interview.New_Sales_Order_From_Account_Page flDemo {get;set;}
public PageReference prFinishLocation {
 get {
PageReference prRef = new PageReference('/' + strOutputVariable); prRef.setRedirect(true);
 return prRef;
 }
 set { prFinishLocation = value; }
 }
public String strOutputVariable {
 get {
 String strTemp = '';
 if(flDemo != null) {
 strTemp = string.valueOf(flDemo.getVariableValue(New_SalesOrder_ID));
 }
 return strTemp;
 }
 set { strOutputVariable = value; }
 }
}
</apex:page>

Which failed so miserably, I couldn't even save it. (Pretty sure my first error was something along the lines of "controller="flowController" doesn't exist."
Is the controller and then this other set of code supposed to be two different pages? or one in the same?
I'd like to reiterate how new I am to all of this, but I'm trying to learn by working off of and disecting good examples like these!
Thanks so much for your help!